Functional gold nanoparticles in diagnosis and treatment of cancer: A systematic review Open Access

Cancer, defined as uncontrolled growth and spread of cells in the body, poses a significant global threat to public health and social development.¹ According to the International Agency for Research on Cancer (IARC), in 2022, around 20 × 10⁶ individuals were diagnosed with cancer worldwide, with 9.7 × 10⁶ cancer deaths reported.² While diagnostic technologies such as computed tomography (CT) scans, x rays, magnetic resonance imaging (MRI), and liquid biopsy play a significant role in cancer diagnosis, some limitations remain.^3–5 For example, CT and x rays involve exposure to ionizing radiation, which may elevate the risk of cancer in patients with prolonged or frequent use.⁶ In addition, the limited sensitivity of these imaging tests for detecting small lesions or early stage lesions can lead to misdiagnosis or underdiagnosis.^3,7 Liquid biopsy, a non-invasive detection method, is promising but has associated challenges, particularly in distinguishing malignant tumors from benign tumors.⁸ In terms of treatment, conventional surgical procedures, chemotherapy, and radiation therapy (RT) have evolved and achieved notable successes; however, they continue to encounter significant challenges.⁹ For instance, it can be challenging to completely remove cancerous tissue with surgical treatment, with residual lesions often being left, increasing the potential for recurrence.¹⁰ Chemotherapeutic drugs generally lack precision, resulting in adverse effects on both cancerous and healthy tissues and causing nausea, hair loss, and immune suppression.¹¹ Radiotherapy also carries inherent risks of exposure to radiation and potential damage to surrounding healthy tissues.¹² There is, therefore, urgent demand for new techniques to address these challenges (Table I).

Recent advances in nanotechnology have yielded novel concepts and methodologies for the diagnosis and treatment of cancer. Integrating nanotechnology into cancer diagnosis and treatment has introduced new possibilities for real-time monitoring of the tumor microenvironment (TME), facilitating the assessment of disease progression and treatment efficacy at an earlier stage. Furthermore, the development of minimally invasive and non-invasive treatment options has been promoted, effectively overcoming the limitations of traditional treatments.¹⁵ Nanotechnology has achieved widespread use in the early diagnosis and treatment of cancer. Gold nanoparticles (Au NPs) are a class of nanomaterials that have garnered significant attention owing to their exceptional physicochemical properties, which render them highly promising for applications in cancer diagnosis and treatment.¹⁶ Their excellent physicochemical stability is a primary factor contributing to their efficacy within the TME.^17,18 The abnormal vascular permeability and absence of effective lymphatic drainage in tumor tissues enable Au NPs to achieve passive targeting of malignant tumor tissues through the enhanced permeation and retention (EPR) effect, thereby improving diagnostic and therapeutic efficacy and reducing the impact on healthy tissues.^19,20 In addition, the facile surface functionalization of Au NPs allows for the preparation of a variety of biosensors with enhanced sensitivity.²¹ For instance, the surface of Au NPs can be modified with recognition elements that specifically bind to particular biological targets, such as antibodies and DNA.²² This results in significant surface-enhanced Raman scattering (SERS), metal-enhanced fluorescence (MEF), and fluorescence resonance energy transfer (FRET) effects, enabling precise detection of target molecules at very low concentrations.^23–25 Au NPs, therefore, offer great potential for application in the field of biomedical detection and testing.

The near-infrared (NIR) light response properties of Au NPs have garnered significant attention owing to the localized surface plasmon resonance (LSPR) effect. Under NIR irradiation, Au NPs exhibit strong light absorption, which efficiently converts light energy into heat energy, leading to a substantial increase in the temperature of local tumor tissues.²⁶ The limited blood supply and poor heat dissipation capacity of tumor tissues result in a notable reduction in their heat resistance. Consequently, the localized high temperature compromises cell membranes and denatures proteins, thereby inducing irreversible damage or even cell death. This strategy is widely employed in photothermal therapy (PTT) for cancer treatment.¹⁶ The chemical modification of Au NPs using drug carriers, such as porous polymers and liposomes, has been shown to enhance the drug loading efficiency. The encapsulated anticancer drugs can then be released from the carriers after external laser irradiation, thereby enabling combined PTT and chemotherapy treatment.^27,28 In addition, a photosensitizer (PS) can be bound to the surface of Au NPs, and then released after the complex is passively targeted to the tumor tissue.²⁷ Under specific wavelength laser irradiation, the PS induces the production of singlet oxygen (¹O₂) or other free radicals through photochemical processes, causing irreversible damage to cancer cell membranes, proteins, DNA, and other biomolecules, and realizing photodynamic therapy (PDT) to achieve effective cancer treatment.²⁹ However, the low oxygen content in tumor tissues and the inconsistent wavelengths of lasers applicable to PTT and PDT limit the efficacy of this combined strategy.^30,31 The expression of hydrogen peroxide (H₂O₂) in cancer cells has garnered significant interest from researchers, as a high concentration of reactive oxygen species (ROS) can attack cellular components, such as lipids, proteins, and DNA, ultimately leading to the death of cancer cells.³² Among the numerous ROS enhancement strategies, chemo-dynamic therapy (CDT) employs the Fenton effect or Fenton-like effect to convert low-activity H₂O₂ into highly toxic hydroxyl radicals (·OH) by increasing the level of ROS in cancer cells to treat cancer.³³ Concurrently, the localized high temperatures generated by PTT can further accelerate the Fenton or Fenton-like effect, thereby enhancing CDT.³⁴ Consequently, the use of Au NPs in early diagnosis and cancer treatment strategies holds considerable promise.

In this review, we provide a concise overview of the optical properties exhibited by Au NPs, including LSPR, SERS, MEF, and FRET. We then proceed to a detailed discussion of the application mechanism of biosensors based on these properties in the detection of cancer markers. The review also includes a comprehensive summary of relevant diagnostic studies from multiple perspectives, accompanied by a critical analysis of the strengths and limitations of various biosensor types. This review further elucidates the deficiencies of several conventional cancer therapies in the context of tumor tissues, highlighting the potential of novel cancer therapeutics based on Au NPs to address these challenges. In this regard, this review presents a range of optical therapies based on Au NPs, with a particular emphasis on the recent advances in PTT, PDT, and combination therapy in cancer treatment (Fig. 1). This review underscores the limitations of employing Au NPs for cancer diagnosis and treatment, while concurrently anticipating the expansion of the applications of Au NPs in diagnostic and therapeutic integration.

Au NPs are broadly useful in biomedical applications owing to their inertness and biocompatibility.³⁵ When subjected to the oscillating electromagnetic field of a light wave, free electrons in the conduction band of metal nanoparticles resonate at the frequency of the light, which produces a collective coherence phenomenon known as the surface plasmon resonance (SPR) effect, which results in strong light absorption.^36,37 When the incident light wavelength is comparable with or smaller than the electron mean free path (∼100 nm) of the Au NPs, and the incident light frequency resonates with the intrinsic frequency of surface plasmon polaritons in the nanoparticles, free electrons on the metal surface couple strongly with the electromagnetic field.^38,39 Under these conditions, the spectrum exhibits a pronounced resonance absorption or scattering peak. This phenomenon is referred to as the LSPR effect of the Au NPs [Fig. 2(a)].⁴⁰ For Au NPs, the LSPR effect is closely related to the nanoparticle morphology, size distribution, chemical composition, particle spacing, and dielectric properties [Fig. 2(c)].^41,42 Typically, this leads to a broadening of the LSPR and a well-defined redshift when the size of Au NPs increases. For example, the Au nanospheres showed a single LSPR peak in the visible region. The dipolar plasmon mode redshifts gradually from 521 to 806 nm, when the average diameter of the Au nanosphere increases from 24 to 221 nm.⁴³ Peak broadening is clearly observed owing to increasing radiative losses for the nanospheres with larger sizes. A similar result was reported by Zheng et al.⁴⁴ The LSPR absorption bands of Au nanocubes have been precisely controlled from 520 to 600 nm with changing edge lengths.⁴⁴ Compared to sphere counterparts, anisotropic Au NPs not only provide abundant LSPR modes but also focus more light to the nanogap and tip. For example, Au nanorod (NR) and nanobipyramid displayed two modes of LSPR, which corresponded to the electron oscillations perpendicular and along the anisotropic Au NPs. Hence, the longitudinal LSPR mode of Au nanorod and nanobipyramid can be continuously shifted from the visible to the NIR region by changing the aspect ratio.^45,46 The Au nanostar is composed of several protruding nanotips and a central core, which usually show multiple LSPR modes corresponding to the tips and core–tip interactions.⁴⁷ For example, compared to seeds, the absorption spectra of nanostars were significantly wider, and the peak redshifted with the increase in the ratio of branch length to core radius.⁴⁸ Decreasing the spacing between particles enhances electromagnetic coupling, resulting in redshifted peaks and changes in intensity.⁴⁹ For example, depending on ratios, a new redshifted absorbance peak from about 600 to 800 nm, with assembly of large and small Au NPs.⁵⁰ The unique structure of the particles induces the formation of hot spots, the distribution density and intensity of which directly affect the spectral absorption and scattering peaks, while significantly enhancing the local electromagnetic field strength.⁵¹ Furthermore, variations in the dielectric constant of the surrounding medium can dynamically modulate the resonance frequency, forming the core mechanism of LSPR sensing technology.^52,53 This provides an ideal experimental platform for molecular-level studies of interactions with optical substances. In biosensing applications, the local electromagnetic field generated by the LSPR effect exhibits extreme sensitivity to minute changes in the refractive index of the surrounding medium. This sensitivity originates from the sub-wavelength optical field effect, which is enhanced by the excitation of metal nanostructures with equipartitioned excitons.⁵⁴ Consequently, LSPR technology demonstrates considerable potential in the field of sensing. The LSPR properties of Au NPs offer substantial potential for applications in biomedical diagnostics, biosensing, photocatalysis, PTT, environmental monitoring, and drug analysis.^55–57 

The most salient property of Au NPs is the preferential absorption of light within the NIR spectrum. This property not only substantially enhances their optical contrast within living organisms but also effectively circumvents the interference of the intrinsic fluorescence of tissue.^58,59 When light interacts with Au NPs, the absorbed energy can be released through various mechanisms, primarily elastic and inelastic scattering processes. In the context of light–matter interactions, scattering phenomena can be categorized into two primary types: Rayleigh scattering and Raman scattering. Under laser irradiation, following the absorption of an incident photon by a medium molecule, the majority of the photons only experience a change in direction, a phenomenon referred to as Rayleigh scattering. In contrast, a minor proportion of the photons undergo an increase or decrease in energy, resulting in a frequency shift, a process known as Raman scattering.^60–62 This distinctive scattering property provides a crucial physical foundation for using Au NPs in biomedicine [Fig. 2(b)].⁶³ 

Raman activity refers to the inelastic scattering of light by molecular vibrations or rotations in a material, which generates spectral features (Raman spectra) containing vibrational fingerprint information characteristic of the molecular structures. This activity originates from the inelastic scattering of Raman-active molecules, which essentially involves the transfer of energy between photons and molecular vibrational modes. The Raman scattering signal in its natural state is extremely weak, a limitation that has been addressed by the introduction of SERS technology.⁶³ SERS is an ultrasensitive analytical method based on vibrational spectroscopy, which can enhance the Raman-scattered light signal by 10⁷–10¹⁴ times, thereby offering significant advantages in the resolution and analysis of complex molecular structures.⁶⁴ The enhancement mechanism of SERS is not yet fully understood; however, two main complementary theoretical models have been proposed: electromagnetic enhancement (EM) and chemical enhancement (CM) [Figs. 3(a) and 3(b)].^65–67 The EM model is predicated on the LSPR effect, which instigates collective oscillations of the free electrons when incident light is directed toward metal nanoparticles.^68,69 This results in the generation of a localized electromagnetic field, thereby substantially amplifying the Raman scattering signal.^70,71 Conversely, CM is associated with the formation of complexes between the adsorbate molecules and the metal surface, which results in electronic coupling between the molecules and the substrate.⁶⁹ This process significantly increases the molecular polarization rate through a charge transfer process, thereby enhancing the Raman signal.⁷² 

As a potent bioanalytical tool, SERS technology has demonstrated significant potential in areas such as disease diagnosis and environmental monitoring due to its ability to substantially enhance Raman signals. Although gold nanomaterials have already shown excellent performance as SERS substrates, researchers have further improved detection sensitivity through a variety of strategies, which primarily include the following. (1) Constructing core–shell structures, anisotropic nanoparticles, and hierarchical 3D structures to enhance the LSPR effect. For example, Liu et al. successfully synthesized sea urchin-shaped Au NPs, whose surfaces feature hundreds of spines that generate a large number of localized electromagnetic field enhancement “hot spots.”⁷³ SERS measurement results indicate that the Raman signal enhancement of a single sea urchin-shaped nanoparticle can reach 7–8 orders of magnitude. Hilal et al. reported a series of 3D gold framework structures, which significantly improved the SERS enhancement effect.⁷⁴ (2) Utilizing self-assembly techniques to construct arrayed SERS substrates with densely distributed plasmonic “hot spots.” For instance, Zhao et al. precisely constructed trimer nanoarrays, achieving spatial matching of probe molecules with “hot spots” regions and applied them to the SERS detection of lung tumor tissue secretions.⁷⁵ Gao et al. prepared gold nanoplate arrays (AuTAG) as SERS substrates by attaching gold nanoplate arrays to the surface of thermoresponsive hydrogels. AuTAG functions as an actively tunable plasmonic device; by controlling the temperature to alter the hydrogel volume, the distance between particles can be adjusted.⁷⁶ (3) Achieving specific capture of target molecules and SERS signal amplification through ligand exchange, molecular imprinting, or bio-probe modification. For example, Jiang et al. achieved selective and sensitive detection of sulfamethoxazole (SMZ) by in situ reducing Ag NPs in the combination of molecularly imprinted polymers (MIP) and SERS.⁷⁷ (4) Developing in situ real-time monitoring micro–nano sensor systems. For instance, Zhou et al. reported a SERS-integrated microfluidic platform that enabled multiplexed analysis and detection of sEV glycans from non-small-cell lung cancer (NSCLC) patients (EV-GLYPH).⁷⁸ Ho et al. enhanced SERS signals through a salt-induced gold nanoparticle aggregation process on a chip in the presence of HER2 aptamers and HER2-positive exosomes, achieving a detection limit of 4.5-log₁₀ particles/ml within a 5 min detection time.⁷⁹ (5) Achieving intelligent recognition and quantitative analysis of SERS signals for ultra-low concentration molecules. For example, Bi et al. invented digital colloid-enhanced Raman spectroscopy (dCERS), utilizing colloidal nanoparticles to achieve efficient quantitative detection of various molecules (such as dye molecules, metabolic small molecules, nucleic acids, and proteins) can be accomplished, with a quantitative detection limit reaching below 1 fM.⁸⁰ Dong et al. established a label-free, liquid-phase optical detection system, using inexpensive silver nanowires as SERS probes. Furthermore, they conducted in-depth data analysis and cancer prediction using artificial intelligence.⁸¹ The high sensitivity, specificity, and detection of molecular information from biological samples exhibited by SERS technology suggest its considerable potential for application in the field of cancer biomarker detection [Fig. 3(c)].^64,82 With the continuous progress of nano-preparation technology and spectral analysis methods, SERS is gradually developing into a multifunctional bioanalytical platform with a broad scope for application in clinical diagnosis, disease screening, and biosensing.⁸³ 

A distinctive interaction between metal nanostructures and fluorophores over a certain distance range (5–90 nm) has been shown to considerably enhance the quantum yield and photostability of fluorophores while concomitantly reducing their lifetime.^84,85 This phenomenon is referred to as MEF. The concept of the MEF effect was first introduced by Geddes in 2002 and encompasses two complementary phenomena: enhanced absorption and enhanced emission.^31,86 The MEF effect is contingent upon the plasmonic coupling mechanism between the metal and the fluorophore. When coupled quanta are radiated from the metal surface, the fluorophore and the metal remain coupled in both the ground and excited states, thereby enhancing the fluorescence intensity hundreds of times.^87,88 Conversely, when the fluorophore is nearby (<5 nm) or in direct contact with the nanometal surface, it undergoes quenching, leading to a quenching effect that exceeds the enhancement effect, resulting in a decrease in fluorescence intensity. The fluorescence intensity of the fluorophore is highly sensitive to its proximity to the metal surface. Therefore, precise control of the distance between the fluorophore and the metal is crucial for optimizing the fluorescence signal intensity in current MEF research.⁸⁹ The size, shape, and distribution density of metal nanoparticles significantly influence their plasma scattering properties. When the absorption spectra of metal nanoparticles overlap with those of fluorophores, the excitation and emission rates of the fluorophores are markedly enhanced, leading to a more efficient fluorescence signal output.^90,91 In the proximity of Au NPs that have been irradiated by incident light, excited fluorescent molecules undergo a pronounced interaction with the local electromagnetic field that is generated. This interaction exerts a dual effect on the fluorescent molecules. On the one hand, it enhances the excitation rate, thereby increasing the fluorescence emission intensity. On the other hand, the structural properties of the Au NPs regulate the radiative decay rate of the fluorescent molecules, thus altering the energy release path.^92,93

In the absence of an external field, non-radiative processes, such as intramolecular vibrational relaxation, often interfere with the radiative decay of fluorescent molecules. This results in a decrease in quantum yield. However, the localized electromagnetic field in the vicinity of Au NPs has been shown to trigger the transfer of excited state energy to the metal surface plasma by non-radiative leaps. This process not only accelerates the radiative decay rate but also significantly increases the fluorescence quantum yield. Furthermore, the accelerated rate of decay reduces the duration of the fluorophore transition from the excited state to the ground state. This mitigates the risk of photo-destruction and augments the photostability of the fluorophore.⁸⁶ In the context of fluorescence technology applications, quantum yield and photostability are pivotal performance indicators, and the MEF effect facilitates concurrent optimization of both.⁹⁴ 

During the process of FRET, the excited state energy of a donor molecule is transferred to an acceptor molecule through dipole–dipole interactions, resulting in a significant decrease in the intensity of the donor fluorescence, also known as a fluorescence burst (Fig. 4). Concurrently, the acceptor molecule emits enhanced fluorescence, a phenomenon referred to as sensitized fluorescence due to the energy gained.⁹⁵ This distinctive energy transfer mechanism renders FRET, a versatile tool with a wide range of applications in biological research, molecular detection, and other fields of study. The FRET effect is contingent upon the fulfillment of specific conditions. (1) There must be a certain overlap area (>30%) between the emission spectrum of the donor fluorescent molecule and the absorption spectrum of the acceptor fluorescent molecule. However, an excessively large overlap area may compromise the accuracy and stability of the FRET.⁹⁶ (2) The distance between the donor and the acceptor must be maintained within 10 nm because the efficiency of the FRET is inversely proportional to the sixth power of the spacing between them, which makes FRET very sensitive to small changes in the distance between molecules. Consequently, reducing the distance between the donor and acceptor has been identified as a strategy for enhancing FRET efficiency.⁹⁶ (3) The relative orientation of the dipole moments of the donor and acceptor molecules is crucial. FRET efficiency is more significantly enhanced when the emission dipole moment of the donor is distributed in parallel with the absorption dipole moment of the acceptor than when they are perpendicular to each other.⁹⁷ (4) The strength of intermolecular forces between the donor and acceptor is a critical factor in ensuring the formation of effective FRET pairs.⁹⁸ FRET technology allows for the non-invasive monitoring of protein interactions in living cells.⁹⁹ The technology is based on the principle of measuring the efficiency of non-radiative energy transfer between donor and acceptor molecules. By measuring this transfer, FRET can detect direct interactions between proteins with high sensitivity, specificity, and low toxicity.¹⁰⁰ The accuracy of the method was further enhanced by the introduction of the quantitative FRET technique. This technique effectively mitigates the interference caused by fluorescence crosstalk and variations in experimental conditions, thereby enabling a more realistic interpretation of the FRET signals to elucidate the intricate mechanisms underlying the interactions between biomolecules.¹⁰¹ 

In 1974, Fleischmann et al. serendipitously discovered an anomalous enhancement of the Raman signal from pyridine molecules adsorbed on a rough silver electrode.¹⁰² They attributed this phenomenon to the increased surface area resulting from the roughened silver surface, which facilitated the adsorption of a greater number of pyridine molecules. However, subsequent research conducted by Jeanmaire and Van Duyne in 1977 revealed that this factor was not a primary contributor to the Raman signal enhancement.¹⁰³ Instead, they ascertained that the signal enhancement was associated with the presence of an as-yet unidentified enhancement mechanism. This observation led to the hypothesis that the signal enhancement was caused by the rough surface of the electrodes, which was subsequently validated through formal confirmation of the SERS phenomenon. The enhancement effect is primarily attributed to the electric field amplification caused by the oscillation of equivalent charges on the metal surface, commonly referred to as the SPR effect. When metal nanostructures are exposed to laser light, the free electrons on the metal surface undergo collective oscillations, known as LSPR. This phenomenon significantly enhances the electromagnetic field near the surface of the metal nanostructures, creating regions referred to as hot spots.¹⁰⁴ Molecules adsorbed on metal surfaces are subjected to an enhanced electromagnetic field, which results in greater polarization of the electron cloud. This, in turn, increases the Raman scattering cross section of the molecular vibrational modes. Consequently, greatly enhanced Raman scattering signals of the molecules are produced, allowing even very low concentrations of marker molecules to be detected.¹⁰⁵ 

In the subsequent decades, SERS technology has been recognized as a powerful tool for identifying cancer markers owing to its features, such as high sensitivity, specificity, rapid analysis, and non-invasiveness.^106–108 In this field, Au NPs have been extensively used as SERS substrates owing to their exceptional physicochemical properties.¹⁰⁹ Moreover, by altering the size, shape, and surface functionalization of Au NPs to modulate their LSPR effect, they can be adapted to diverse detection requirements [Fig. 5(a)]. To date, researchers have developed two SERS detection methods: direct detection (label-free) and indirect detection (label-based) [Figs. 5(b) and 5(c)].

Label-free SERS eliminates the need for Raman tags in the detection of biological samples. This approach directly captures the Raman spectral fingerprints of the samples, thus bypassing the complex labeling process. It is characterized by simplicity, rapidity, cost-effectiveness, and the ability to detect a variety of analytes, making it suitable for early cancer detection.^110,111 As pivotal messengers of intercellular communication, exosomes convey substantial information about biomarkers, including proteins and nucleic acids.¹¹² Consequently, their thorough analysis is paramount for understanding disease states. Raj et al. used the western blot (WB) technique to identify urinary exosomal markers, including tumor susceptibility gene 101 (TSG101), apoptosis-linked gene 2 interacting protein X (ALIX), and cluster of differentiation 9 (CD9).¹¹³ In addition, they detected other common proteins such as aquaporin-2 (AQP2) and Na–K–Cl cotransporter isoform 2 (NKCC2). However, traditional exosome assays, such as WB and enzyme-linked immunosorbent assay (ELISA), are characterized by complex operation, the potential for false positives, and low sensitivity, which collectively limit their utility in large-scale clinical applications.^114,115 Considering the limitations mentioned, Dong et al. developed an Au-coated TiO₂ microporous inverse opal (MIO) structure, inspired by a honeycomb, which demonstrates a significant “slow light effect” and efficiently captures exosome particles in the blood [Fig. 6(a)].¹¹⁶ By analyzing the SERS signal at 1087 cm⁻¹ in plasma, the protein phosphorylation status of exosomes can be determined, serving as a potential indicator for tumor liquid biopsy in cancer detection [Fig. 6(b)]. Carmichael et al. successfully used the SERS technique for label-free characterization of exosomes.¹¹⁷ They distinguished exosomes from various sources by comparing their SERS spectra, employing principal component discriminant function analysis (PC-DFA). Furthermore, the method demonstrated high predictive accuracy when applied to serum samples from pancreatic cancer patients. The available studies confirm the high value of exosomes as potential cancer biomarkers. The simplicity and high efficiency of the Au NPs-based SERS non-invasive diagnostic method for label-free detection demonstrates the promising application of this technology in cancer diagnosis.

Biomolecules, such as proteins, are typically composed of amino acids that have small Raman scattering cross sections, resulting in weak SERS signals during direct detection, which adversely affects detection sensitivity. To enhance both the sensitivity and specificity of detection, Raman labeling-based indirect detection has been developed.¹⁰⁸ Raman tags based on Au NPs consist of Raman reporters and surface biofunctionalized modification molecules. The Au NPs enhance the SERS signal of the Raman reporters immobilized on their surfaces, which exhibit a large Raman scattering cross section. The surface biofunctionalized molecules specifically recognize and bind to target detectors. In addition, Raman tags can protect the shell layer to prevent the shedding of Raman reporters or facilitate the surface functionalization of metal nanoparticles.¹¹⁰ Common Raman reporters include 4-mercaptobenzoic acid (4-MBA), Rhodamine 6G (R6G), crystal violet (CV), 4-mercaptopyridine (4-Mpy), and 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB).

For instance, Zhang et al. developed a SERS tag composed of Au–Ag/Au core–shell plasmonic nanorods with tunable nanogaps that contain Raman reporters.¹¹⁸ The presence of such nanogaps allows Raman reporters, such as 4-Mpy and R6G, to generate enhanced, stable, and reproducible intra-gap SERS signals. Following the functionalization of the material with a specific mucin 1 (MUC1) aptamer, it can be used for the specific detection of Michigan Cancer Foundation-7 (MCF-7) in a simulated blood environment. However, traditional Raman-labeled SERS detection techniques tend to be highly specific and can only detect a single target marker, limiting the flexibility and applicability of the test. Numerous studies have been conducted to overcome these limitations. Wu et al. proposed a 3D hierarchical assembly cluster-based SERS strategy for ultrasensitive and quantitative analysis of two upregulated miRNAs (miR-21 and miR-31) associated with colorectal cancer (CRC).¹¹⁹ This strategy used Au nanocage@Au nanoparticles (Au NC@Au NPs) as SERS probes, silver-coated Fe₃O₄ magnetic nanoparticles as magnetic capture units, and signal amplification probes [Fig. 6(c)]. The method demonstrated excellent specificity and interference immunity, successfully differentiating clinical serum samples from CRC patients and healthy individuals. In addition, it provided guidance for tumor, lymph node, and metastasis (TNM) staging by monitoring changes in miR-21 and miR-31 levels, offering the potential for early diagnosis and disease monitoring [Fig. 6(d)]. Wang et al. proposed three innovative techniques based on the Au NF@SiO₂ structure to develop a multicolor cocktail of bioorthogonal SERS nanoprobes for the targeted diagnosis of multiple breast cancer phenotypes.¹²⁰ By modifying the nanoprobes with various functional groups, multiple specific markers on breast cancer cells could be identified and imaged. The nanoprobes, equipped with target ligands, bound specifically to tumor cell biomarkers such as nucleolin, integrin α_vβ₃, and CD44, thereby enabling the detection of multiple phenotypic characteristics of tumor cells. However, Raman-labeled SERS detection presents several challenges, including the complexity of preparing Raman labels, high costs, and potential interference from interactions between Raman labels and target molecules. These limitations can be mitigated to a certain extent by targeted construction of novel structures based on Au NPs. This method, therefore, holds significant promise for cancer diagnosis.

When a molecule is excited, the electrons in the excited state transition to the ground state via radiative decay, thereby emitting photons. Alternatively, the electrons may transition to other states through non-radiative decay, a process that does not involve photon emission. The presence of metal nanostructures in proximity to fluorescent molecules has been shown to induce changes in the rates of radiative and non-radiative attenuation, thereby modulating the fluorescence intensity and lifetime of the molecules. The degree of enhancement or quenching of fluorescence exhibited by metal nanostructures has been found to be intimately associated with the distance separating the fluorescent molecule from the metal nanostructure. The MEF phenomenon occurs in the range 5–90 nm, resulting in a significant enhancement of fluorescence intensity, reaching up to hundreds of times the initial level.⁸⁴ However, when the distance between the fluorescent molecules and the surface of the Au NPs is minimal, specifically less than 5 nm, or when the fluorescent molecules are in direct contact with the Au NPs, a process known as FRET can take place. The energy transfer from the fluorescent molecules to the Au NPs is facilitated by the LSPR effect of the Au NPs, leading to quenching of the fluorescent molecules.¹²¹ This property is exploited by immobilizing fluorescently labeled antibodies with specific recognition and binding effects on Au NPs. When targeted to recognize and bind to specific tumor marker molecules, the resulting fluorescence intensity can be detected for cancer diagnosis.¹²² 

The use of fluorescently labeled antibody technology for cancer marker detection was first reported in the 1970s and 1980s. For instance, Koprowski et al. conducted research in this area that laid the foundation for subsequent fluorescence imaging techniques and cancer marker detection.¹²³ Some endogenous fluorophores present in tissues and cells—including lipofuscin (LF), nicotinamide adenine dinucleotide hydride (NADH), and flavin adenine dinucleotide (FAD)—exhibit autofluorescence when exposed to light of specific wavelengths.^124–127 As tumors grow, intracellular metabolic activities undergo changes, which are directly reflected in the metabolic activity, concentration, and spatial distribution of endogenous fluorophores, such as NADH and FAD.¹²⁸ Consequently, analysis of the autofluorescence spectra of these fluorophores can allow cancer diagnosis. Ostrander et al. demonstrated that optical redox ratios are positively correlated with estrogen receptor (ER) expression.¹²⁹ The researchers used confocal microscopy to collect fluorescence emission from NADH and FAD and then calculated the optical redox ratios. They observed that the optical redox ratios were elevated in the ER(+) breast cancer cell lines compared to the ER(−) cell lines, thereby demonstrating the potential for distinguishing between normal breast epithelial cells and breast cancer cell lines using the optical redox ratio. Furthermore, the expression level of estrogen receptor 1 (ESR1), as determined by real-time quantitative PCR, exhibited a direct correlation with the optical redox ratio (Pearson’s correlation coefficient = 0.8122, P = 0.0024), which provides additional validation for the underlying theory. The advantages of this technique include non-invasiveness, high sensitivity, safety, and simplicity, and it has demonstrated success in cancer diagnosis. However, several limitations are also evident. The study primarily focused on breast cancer cell lines, specifically distinguishing between ER-positive and ER-negative subtypes, and the validity of the technique has not been fully verified for other cancer types or different biological subtypes of breast cancer. In addition, the presence of other endogenous fluorophores may interfere with experimental results and complicate deep-tissue imaging.¹³⁰ 

The introduction of exogenous fluorophores with optimal specificity, targeting, and biocompatibility has the potential to address these issues to a considerable degree. Therefore, Au NPs can be considered to have significant promise for future applications. Wu et al. designed a dual-aptamer-functionalized gold nanoprobe (DA-GNP), which had fluorescently labeled estrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2)-specific aptamers attached to the surface of Au NPs [Fig. 7(a)].¹³¹ The probe achieved fluorescence quenching based on FRET, and fluorescence signals were restored when the aptamers bound specifically to their corresponding biomarkers [Fig. 7(b)]. This enabled quantitative analysis of different breast cancer subtypes. In the experiment, TAMRA-tagged aptamers were used for HER2, while FAM-tagged aptamers were used for ER. The results showed that enhanced TAMRA (red) fluorescence was observed in HER2-positive SK-BR-3 cells, while enhanced FAM (green) fluorescence was seen in ER-positive MCF-7 cells. Moreover, in the two-color mode, DA-GNP successfully differentiated three breast cancer cell lines: MDA-MB-231 (basal-like, ER−, HER2−), SK-BR-3 (HER2-enriched, ER−, HER2+), and MCF-7 (luminal-A, ER+, HER2−). MTT assay confirmed the biocompatibility of DA-GNP and determined the optimal concentration (0.3 nM) and incubation time (30 min). Furthermore, Lv et al. developed an innovative Au NPs@COF core–shell nanoprobe containing Cy5-labeled MUC1 aptamer (red channel), FITC-labeled DNA hairpin responsive to miR-21 (green channel), and a molecule that responds to ROS.¹³² The probe was simultaneously detected by fluorescent and SERS signaling, enabling the precise detection of three cancer biomarkers (MUC1, miR-21, and ROS) in living cells [Fig. 7(c)]. In the absence of the target biomarker, the fluorescence is quenched; upon specific binding to the target biomarker, the corresponding fluorescence signal recovers and generates the SERS signal, thus enabling optical imaging of the biomarkers. This technology provides a new tool for cancer diagnosis and drug screening. As demonstrated above, fluorescent sensors based on Au NPs hold considerable promise for detecting cancer markers. Furthermore, the combined application of multiple cancer diagnostic methods shows significant potential for future advances.

When light is incident on Au NPs, LSPR is observed when the frequency of the incident light matches the overall vibrational frequency of the Au NPs, which results in the strong absorption of photon energy by the nanoparticles.¹³³ The enhancement of the local electromagnetic field and the production of strong resonant absorption peaks due to this interaction lead to the display of a specific color in the Au NPs solution.¹³⁴ The LSPR absorption peak is influenced by the shape, size, and interparticle distance of Au NPs. Consequently, a significant color change can be observed when the shape or size of Au NPs changes or when aggregation occurs between particles.¹³⁵ In addition, Au NPs have extremely high extinction coefficients, which allow for the analysis of specific biomolecules by using colorimetric methods.¹³⁶ Since the pioneering study by Valadi et al. demonstrated the significant role of exosomes in intercellular communication, numerous studies have corroborated their potential as non-invasive biomarkers for cancer monitoring and diagnosis.¹³⁷ Conventional colorimetric assays are not generally preferred for tumor marker detection owing to their limited specificity and sensitivity. Nevertheless, they can serve as supplementary tools for identifying certain biomarkers. Although not a traditional colorimetric method, ELISA employs color changes for quantitative analysis.¹³⁸ In an ELISA, specific antibodies capture protein markers on exosome surfaces within the sample.¹³⁹ Subsequent enzymatic reactions generate a color signal measurable by colorimetry. The limitations of ELISA include low detection limits, non-specific binding, and complex procedures.¹⁴⁰ 

Au NPs exhibit catalytic activity similar to that of natural enzymes, allowing them to mimic enzymatic functions.¹⁴¹ Consequently, Au NPs are used in colorimetric biosensors for detecting proteins in exosomes.¹⁴² These biosensors enable the analysis of specific protein markers in the exosomal environment, aiding in cancer diagnosis. Based on this property, Jiang et al. developed a novel colorimetric sensor for rapid, highly sensitive, and specific detection of proteins on the surface of exosomes.¹⁴³ This sensor was created through a noncovalent combination of a set of specific aptamers with 13 nm Au NPs. In high-salt solutions, the aptamer prevents aggregation of the Au NPs. When exosome samples are introduced, the specific binding of the aptamer to the exosome surface proteins supplants its non-specific binding to the Au NPs, leading to aggregation of the Au NPs and a subsequent color change. The quantitative analysis of this color change can be performed by either visual observation or UV–vis spectroscopy. This method facilitates the detection of several surface proteins on exosomes, including CD63, PTK7, PDGF, PSMA, and EpCAM11, across various cell lines such as PC-3, CEM, HeLa, and Ramos.^141,144 Colorimetric methods alone may not attain sufficient specificity and sensitivity for cancer diagnosis. Therefore, they are frequently employed in conjunction with other diagnostic techniques. Wang et al. innovatively designed an FND@GNP dual-nanoparticle system, which immobilizes specific aptamers or antibodies on the surface of Au NPs and uses artificial intelligence (AI) algorithms to improve the accuracy of the integrated assay, including the visual results and fluorescence spectra from lateral flow analysis (LFA).¹⁴⁵ In the presence of the target microRNA, the aggregation of Au NPs resulted in a solution color change from red to blue or purple, while the fluorescence signal of FNDs was enhanced. This synergistic effect of visual observation and spectroscopy led to a substantial improvement in the sensitivity and specificity of the assay. The method achieved a detection limit at the fM level within 5 min with good linearity (R² ≈ 0.9916). A substantial body of research, including the study described, has substantiated the considerable promise of combined assays in the area of cancer diagnosis (Table II).

As demonstrated above, the continuous development of science and technology has led to significant advances in cancer diagnosis, characterized by increased accuracy and innovation. However, the global incidence and mortality rates of cancer remain high, underscoring its persistent threat to public health. This indicates the urgent need for the development of effective and advanced cancer therapies.

The advent of nanotechnology has precipitated a paradigm shift in the field of cancer treatment, with minimally invasive treatment modalities such as PTT emerging as the prevailing standard. Tumor cells exhibit a heightened sensitivity to heat as a consequence of their unique vascular structure. Elevated local temperatures, in the range of 40–43 °C, trigger a series of harmful events, including protein denaturation and cell membrane rupture, ultimately resulting in irreversible damage or even cell death.^146–148 The fundamental principle of PTT involves the use of a photothermal agent (PTA), which facilitates the conversion of light energy into heat energy when exposed to an external light source. This process results in a substantial increase in the temperature of the local tumor tissues, thereby inducing the death of tumor cells through thermal ablation.^148,149 The merits of this approach include its high degree of selectivity, non-toxic nature, and ease of implementation.^150–152 Consequently, PTT has emerged as a prominent area of interest within the medical field. However, the efficacy of PTT is contingent on the photothermal conversion efficiency of the PTA, underscoring the critical importance of selecting an optimal PTA for a given application. Nanomaterials exhibit significant potential in this field owing to their distinct NIR response properties, making them highly regarded by researchers.¹⁵³ Au NPs, in particular, possess LSPR properties and demonstrate high absorption in the NIR region. This enables them to efficiently convert light energy into heat, generating localized high temperatures capable of destroying cancer cells.¹⁵⁴ Furthermore, NIR light can penetrate deeply into tissues and pose minimal harm to healthy human tissue, underscoring the advantages of PTT based on Au NPs in cancer treatment.¹⁵⁵ In PTT, the photothermal conversion efficiency of Au NPs varies with their morphology, with the presence of multiple tips on the nanoparticle being a factor associated with higher efficiency.¹⁵⁶ Xu et al. successfully synthesized monodisperse gold bipyramids (Au BPs) with high morphology yield using cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) [Fig. 8(a)].¹⁵⁷ These Au BPs exhibited very high absorbance in the NIR region with excellent photothermal conversion efficiency. In the subsequent laser on/off experiments, and in vitro and in vivo experiments, the polyethylene glycol (PEG)-functionalized Au BPs demonstrated good photothermal stability and biocompatibility and could be used for high-contrast CT imaging and PTT. This study proposes a simple method for synthesizing Au BPs with high morphology yields and provides a novel material for cancer diagnosis and treatment, which is very encouraging. The high photothermal conversion efficiency, small size, and ease of synthesis of Au nanostars (Au NSs) have also garnered the attention of researchers.¹⁵⁸ Depciuch et al. demonstrated that the photothermal properties of Au NSs are contingent on the edge width and length of the star arms.¹⁵⁹ Furthermore, they found that the photothermal conversion efficiency increases in proportion to increasing arm length, which correlates with the concentration of the reducing agent. However, Au NSs are poorly stabilized, an issue that can be addressed by adding stabilizers to their surface.¹⁵⁸ 

Despite the demonstrated efficacy of various morphologies of Au NPs in PTT, their overall photothermal stability remains insufficient. Furthermore, relying solely on the EPR effect for passive targeting of tumors often leads to inadequate accumulation of Au NPs in tumor tissues, significantly restricting their clinical therapeutic applications.¹⁶⁰ The facile functionalization of Au NPs surfaces allows the attachment of targeting ligands, such as antibodies and DNA fragments, which specifically bind to cancer cell biomarkers, enhancing the targeting capabilities of these materials.¹⁶¹ Zelasko-Leon et al. immobilized MUC1 antibodies and albumin on the surface of gold nanorods (Au NRs) using polydopamine (PD) as a primer, forming a stable, biologically inert, and functional surface.¹⁶² This modification enabled the selective targeting of the MUC1-positive breast cancer cell line MCF-7 and the squamous cell carcinoma cell line SCC-15, significantly reducing cell viability under NIR irradiation (p < 0.005) [Fig. 8(b)]. However, the treatment was ineffective against MUC1-negative MDA-MB-231 breast cancer cells, suggesting a novel targeting strategy for Au NPs-mediated PTT. By modifying the surface of Au NPs with specific biological ligands, the accumulation of Au NPs in tumor tissues was effectively enhanced, greatly improving the therapeutic efficacy of PTT and demonstrating its significant potential for clinical application.

Chemotherapy is a prevalent cancer treatment modality; however, it has several drawbacks that diminish its efficacy, including high toxicity, poor targeting, limited use, and significant systemic side effects.¹⁶³ Consequently, research into targeted delivery strategies for chemotherapeutic drugs remains ongoing. Au NPs have emerged as a promising solution as their surfaces can be easily modified, allowing the effective combination of chemotherapeutic drugs, enhanced drug targeting, and substantial improvement of the therapeutic effect through synergistic PTT and chemotherapy treatment, which offers a novel approach to cancer treatment.^164,165

Sun et al. developed Au NP-coated Pluronic-b-poly(L-lysine) nanoparticles (Pluronic-PLL@Au NPs).¹⁶⁶ They showed that paclitaxel (PTX)-loaded Pluronic-PLL@Au NPs combined with NIR irradiation, effectively inhibited tumor growth, reduced tumor volume, and did not cause significant changes in body weight or organ damage. These findings confirm the biocompatibility of the treatment and its ability to efficiently target tumors. This study highlights the potential of combining PTT with chemotherapy for future therapeutic strategies. Ren et al. developed Au@MPA-PEG-FA-PTX, a gold nanoparticle drug delivery system modified with folic acid (FA) and PEG and carrying PTX.¹⁶⁷ This system exhibited favorable dispersion and homogeneity, and induced apoptosis in cancer cells by increasing intracellular ROS levels. It demonstrated significant inhibition of cancer cells, such as HeLa, exhibiting superiority to the effect of PTX alone. In vivo experiments further substantiated its anti-tumor properties, underscoring the considerable therapeutic potential of the system in cancer treatment. Wang et al. designed gold nanoparticles (Au NVs) with virus-like spines, which primarily entered cells via macropinocytosis and were transported to the Golgi/endoplasmic reticulum system via Rab11-regulated pathways.¹⁶⁸ Ultimately, the Au NVs exited through circulating endosomes, achieving efficient transcytosis and deep tumor penetration compared to conventional Au NPs. Furthermore, when exposed to NIR light, mitoxantrone (MTO), a chemotherapeutic agent delivered by Au NVs, exhibited enhanced photothermal chemotherapeutic efficacy, leading to substantial tumor growth inhibition in a mouse model of CRC (Fig. 9). This study underscores the efficacy of virus-like structures as drug carriers and provides a novel approach to cancer therapy. Synergistic treatment with PTT and chemotherapy based on Au NPs has been shown to yield superior therapeutic outcomes compared to PTT monotherapy. Nevertheless, following repeated NIR irradiation, some of the Au NPs may experience a decline in their photothermal conversion performance.¹⁵⁸ In addition, the drug-carrying capacity of Au NPs has been found to be inadequate, thereby constraining their potential application as drug carriers.¹⁶⁹ 

PDT requires a PS, oxygen, and NIR light.¹⁷⁰ In tumor tissues irradiated by specific wavelengths of NIR light, the PS catalyzes the conversion of oxygen into ROS, which oxidizes critical biomolecules, such as nucleic acids, proteins, and lipids. This results in changes in cell signaling cascades and gene expression, culminating in cell apoptosis.¹⁷¹ The extremely short diffusion distance (<50 nm) of ROS in tissues and cells necessitates the use of PS to achieve precise targeting against tumor tissues.¹⁷² However, most PSs currently in clinical use are hydrophobic, thereby limiting the clinical application of conventional PDT.¹⁷³ The use of Au NPs as a carrier for PS can facilitate passive tumor targeting through the EPR effect. It can also address the challenge posed by PS hydrophobicity, thereby enhancing targeting and biocompatibility. Furthermore, under NIR irradiation, the high temperature generated by PTT triggered by Au NPs has the potential to accelerate the PS release rate and synergistically enhance the PDT, thereby further improving the cancer therapeutic effect.¹⁷⁴ 

Commonly used PSs include chlorin e6 (Ce6), zinc phthalocyanines (ZnPcs), and aluminum phthalocyanine chloride (AlPcS₄Cl). Among these, Ce6 is a second-generation PS known for its high efficacy and low toxicity, making it one of the most widely used PSs.^175–177 García Calavia et al. synthesized Au NPs with a mean diameter of ∼4 nm and combined them with ZnPcs of two carbon chain lengths (C3Pc and C11Pc).¹⁷⁴ The results showed that C11Pc had greater fluorescence in solution than C3Pc, but the combination of C3Pc with Au NPs not only enhanced the fluorescence but also produced more ¹O₂, which significantly improved the therapeutic effect on SK-BR-3 breast cancer cells. This study confirms that the combination of Au NPs with PS can effectively enhance the performance of PS. Castilho et al. developed bifunctional theranostic nanoprobes (BN) for the treatment of triple-negative breast cancer (TNBC).¹⁷⁸ The nanoprobes consisted of 21 nm Au NPs, Ce6, and epidermal growth factor (EGF). At a BN concentration of 0.2 µg/ml, 86% of TNBC cells were killed, with 58% undergoing necrosis and 38% undergoing apoptosis. In addition, the intracellular ROS levels in the treated cells were nine times higher than in normal cells [Fig. 10(a)]. The nanoprobe demonstrated efficient targeted killing of human breast cancer cells during PDT while exerting minimal effects on normal human cells. Li et al. developed a novel gold nanocluster-based platform for the precise targeting of pancreatic ductal adenocarcinoma (PDAC).¹⁷⁹ The platform combines the active ligand U11 peptide, enzyme-triggered 5-ALA, and CTSE-sensitive elastin dye Cy5.5, with confocal laser-endoscopic guidance, enabling targeted PTT/PDT. This approach significantly enhanced therapeutic efficacy while minimizing side effects. However, the low oxygen content in the TME often limits the effectiveness of PDT.¹⁸⁰ To address this issue, Yin et al. developed a light-triggered photosynthetically engineered bacterium (Bac@Au–Ce6) by combining Ce6 and Au NPs [Fig. 10(b)].¹⁸¹ This system effectively targeted tumor tissues and released oxygen through photosynthesis under laser irradiation, thus increasing the oxygen levels in the TME. The enhanced oxygen content promoted the generation of ¹O₂, while PTT further contributed to cancer cell destruction. The experimental results demonstrated that the composite system significantly enhanced the combined therapeutic effects of PDT and PTT, offering a novel platform for multimodal anticancer therapy.

To enhance its efficacy, PTT is generally combined with chemotherapy. However, the high toxicity of chemotherapeutic drugs and the resistance of certain cancer cells inevitably impact the actual clinical effect.^182–184 Moreover, the combination of PTT and PDT is often constrained by the different laser wavelengths required for the two therapies, and the hypoxic environment in the TME can further inhibit the effect of PDT, necessitating an alternative approach.^31,185 CDT uses nanomaterials to catalyze the Fenton or Fenton-like effect, which converts the higher concentration of H₂O₂ in tumor tissues into highly toxic ·OH, leading to a high concentration of ROS.¹⁸⁶ The elevated levels of ROS can attack biological macromolecules, including nucleic acids, proteins, and lipids. This attack can lead to irreversible cell damage or even cell death. Therefore, it can be concluded that cancer treatment can be achieved by increasing the concentration of ROS in cancer cells.^187,188

Fe²⁺-based nanomaterials, including Fe₃O₄ nanoparticles, Fe₂P nanorods, and FePS₃ nanosheets, are frequently used nanocatalysts in CDT.¹⁸⁹ Du et al. developed hollow Fe₃O₄ magnetic clusters (MCs) from solid Fe₃O₄ photonic crystals (PCs) through an Ostwald ripening transition.¹⁹⁰ These hollow MCs demonstrated enhanced magnetic hyperthermia efficiency and peroxidase activity. In vivo experiments revealed that the significant production of ·OH not only induced apoptosis in cancer cells, but also reduced the expression of heat shock proteins, facilitating low-temperature-mediated magnetic hyperthermia (MH). Furthermore, the localized high temperature generated by MH enhanced CDT, resulting in a remarkably high overall tumor suppression rate. Because Fe²⁺-based nanomaterials generally require complex synthesis, Sun et al. developed a novel one-pot method for synthesizing noble-metal@Fe_xO_y core–shell nanoparticles using a redox self-assembly strategy.¹⁹¹ This method was used in tumor therapeutic experiments, with AuPd@Fe_xO_y NPs serving as a model system. The activated AuPd@Fe_xO_y NPs demonstrated a photothermal conversion efficiency of up to 63.5%, which further promoted the generation of ·OH and led to synergistic enhancement with CDT. In addition, the material could be used for CT imaging and the modulation of the TME, providing a straightforward method for synthesizing multifunctional nano-diagnostic agents. Mn²⁺ materials can be used similarly to Fe²⁺-based nanomaterials. Tao et al. synthesized Mn-doped Prussian blue nanoparticles (MnPB NPs), which exhibited strong absorption in the NIR region, thus enhancing the Fenton-like effect induced by Mn²⁺.¹⁹² This effect, in combination with three-modal imaging demonstrated in both in vivo and in vitro experiments, further underscores the significant potential of MnPB NPs for the integration of cancer diagnosis and treatment.

Fe²⁺-based nanomaterials demonstrate high catalytic activity at low pH (pH 2–3); therefore, the TME (pH ∼6.5) is not conducive to optimal catalytic effects.¹⁹³ Conversely, the Fenton-like effect catalyzed by Cu⁺ can be readily executed within the TME (Table III). However, Cu⁺ exhibits significant instability, oxidizing to Cu²⁺ when exposed to air; therefore, stabilizers must be incorporated to maintain its integrity.¹⁹⁴ The distinctive lattice structure of Cu_2−xSe serves to stabilize Cu⁺, while Se, a crucial trace element in human biology, enhances the biocompatibility of the material.^195,196 The study demonstrated that hollow Cu_2−xSe with octahedral morphology can effectively facilitate cancer therapy through PTT and CDT under NIR irradiation.¹⁹⁷ In addition, the chiral structure of the material significantly influences its therapeutic efficacy.¹⁹⁸ Zhang et al. developed a core–shell structure, Au@Cu_2−xSe, which achieved a photothermal conversion efficiency of 56.6% through the LSPR coupling effect [Fig. 11(a)].¹⁸⁹ The high temperature generated by PTT, triggered by NIR irradiation, synergistically enhanced CDT, producing a significant amount of ·OH and inducing apoptosis in cancer cells [Fig. 11(b)]. In addition, the material can serve as a contrast agent in photoacoustic (PA) and CT imaging, thereby improving therapeutic precision. The bioinert-ness of Au NPs enhances the biocompatibility of the composite, while the EPR effect improves tumor tissue targeting. Wang et al. designed Cu_2−xSe-Au Janus nanoparticles that integrate photocatalytic (PCT) activity based on synergistic PTT and CDT [Fig. 11(c)].¹⁹⁹ The nanoparticles exhibited a high photothermal conversion efficiency of up to 67.2% and significantly improved H₂O₂ use efficiency. In vivo experiments demonstrated a ∼95.7% tumor inhibition rate, indicating a strong tumor suppression effect and high-contrast multimodal imaging capabilities. It has been shown that Cu²⁺, generated during treatment through the Fenton-like effect, initially accumulates in the liver and spleen in a mouse model. It is then degraded and metabolized through enterohepatic pathways and is almost completely gone at 15 days post-administration.²⁰⁰ 

Preparing size- and shape-controlled Au NPs is important for utilizing them in biological and medical activities. Au NPs are typically synthesized using either chemical or physical approaches. Common preparation methods include the Turkevich, Brust–Schiffrin, seed growth, green synthesis, and laser ablation methods. The Turkevich method involves the reduction of HAuCl₄ using citrate, which not only ensures the biocompatibility of the resulting Au NPs due to its low toxicity but also stabilizes them through electrostatic repulsion to prevent aggregation.²⁰¹ In addition, reducing agents such as amino acids, NaBH₄, and ascorbic acid (AA) may be employed as alternatives to citrate.²⁰² Annadhasan et al. prepared the Au NPs by an environmentally benign method using N-cholyl-l-valine (NaValC) as a self-reducing as well as stabilizing agent in aqueous medium.²⁰³ Ye et al. successfully synthesized Au NRs for PTT by reducing HAuCl₄ with sodium NaBH₄ and AA. CTAB was employed as a stabilizing agent to inhibit the aggregation of the Au NRs.²⁰⁴ The Brust–Schiffrin method utilizes NaBH₄ to reduce HAuCl₄, facilitating the synthesis of stable Au NPs in a biphasic solution.²⁰⁵ Munsell et al. applied this technique to perform a phase-transfer catalytic reduction of HAuCl₄ with NaBH₄ in the presence of tetraoctylammonium bromide (TOAB), yielding functionalized Au NPs. Within this system, 1-pentanethiol (C5-SH) acted as a ligand, providing thiol groups to ensure high monodispersity of the Au NPs.²⁰⁶ Seed-mediated growth is a technique used to produce nanoparticles with controllable morphologies by adjusting the conditions of the growth solution. This method is particularly suitable for synthesizing anisotropic Au NPs, including nanorods, nanobipyramids, and nanoflowers. Jia et al., operating under pH 7.5 conditions, prepared periodically arranged Au NPs on a quartz substrate to serve as seeds. These seeds were then immersed in a growth solution containing HAuCl₄ and Good’s buffer (EPPS). By varying the concentration of EPPS, they successfully fabricated large-area, uniform, spiky Au NP arrays exhibiting strong SERS effects. This approach enabled precise control over the morphology and performance of the Au NPs, making them suitable for various plasmonic applications.²⁰⁷ Green synthesis methods are environmentally friendly as they reduce or avoid the use of organic solvents, thereby minimizing environmental pollution. Utilizing water-soluble carboxylatopill[5]arenes (CP[5]) as stabilizers, Li et al. have successfully developed an environmentally friendly synthesis method for Au NPs in an aqueous medium. This approach eliminates the need for organic solvents, thereby minimizing environmental pollution, and results in Au NPs with exceptional dispersibility and a narrow particle size distribution of 3.1 ± 0.5 nm.²⁰⁸ In their study, Wang et al. prepared monodisperse Au NPs coated by a nanoporous layer of carboxylatopill[6]arenes (CP[6]) in situ through a reversed Turkevich method. In this process, CP[6] acted as both a reducing agent and stabilizer.²⁰⁹ While the laser ablation technique allows precise control over the size of Au NPs by varying laser intensity, enabling the modulation of their optical properties, it is often unsuitable for biomedical applications.²¹⁰ In addition to the aforementioned methods, Wang et al. introduced a nanozyme-powered cup-shaped nanomotor (GNCs-Pt-ICG/Tf) via a facile bottom-up method for enhanced synergistic PDT/PTT upon NIR laser irradiation.²¹¹ Wang et al. successfully synthesized gold nanohorns (Au NHs) using Pichia pastoris cells (PPCs) as templates, through a microbial-mediated method in the presence of hexadecyltrimethylammonium chloride (CTAC).²¹² The synthesis of small-sized, highly stable, and biocompatible Au NPs remains a challenging endeavor. Commonly used stabilizers, such as CTAB and CTAC, can compromise biological membranes during nanoparticle preparation, resulting in irreversible cellular damage and, in severe cases, cell death due to their high cytotoxicity. To mitigate these adverse effects, centrifugation and washing processes are employed to remove excess CTAB or CTAC from the nanoparticle surface, thereby enhancing biocompatibility and reducing cytotoxicity.⁴⁵ In addition, Corma and Lee, among others, used NaBH₄ to reduce HAuCl₄ and employed Cucurbit[n]urils (n = 5–8) as stabilizers to obtain stable, small-sized Au NPs. They successfully encapsulated Au NPs (≤1.0 nm) within CB[7]s.^213,214 Compared to CTAB and CTAC, CB[n]s (n = 5–8, 10, 13–15) possess extremely high biocompatibility, chemical and thermal stability, and unique host–guest properties. They can effectively bind to Au NPs, preventing aggregation and enabling long-term solution stability, making them excellent stabilizing agents. Furthermore, by adjusting the type and concentration of CB[n]s, the morphology and size of Au NPs can be precisely controlled.¹⁴ 

Au NPs offer versatile surface modification capabilities, allowing for the coating of inorganic materials such as silica and selenium, or the application of organic molecules such as SH-PEG, metal-organic framework (MOF), and polyvinylpyrrolidone (PVP). These modifications are employed to improve biocompatibility.⁴⁵ SiO₂ can form a protective shell on the surface of Au NPs, effectively isolating CTAB or CTAC from the biological environment. This encapsulation enhances the biocompatibility of the material. Furthermore, the porous morphology of SiO₂ allows for drug loading, enabling multimodal combination therapies. Cui et al. successfully coated Au NRs with SiO₂, producing Au NRs@mSiO₂ and Au NRs@mSiO₂@LB. Cytotoxicity tests revealed a cell survival rate of 94% at a concentration of 100 µg/ml, demonstrating excellent biocompatibility.²¹⁵ With Au NPs serving as the core and a Cu_2−xSe shell coating, the LSPR coupling effect between Au and Cu can be leveraged to enhance photothermal conversion efficiency. In addition, Se, a vital trace element for human health, contributes to improving the material’s biocompatibility. Studies have demonstrated that, under low concentration conditions, the material exhibits a high cell survival rate in cytotoxicity tests.^189,199 SH-PEG binds to Au NPs through covalent Au–S bonds, effectively preventing nanoparticle aggregation through spatial steric hindrance. In addition, SH-PEG improves biocompatibility and extends circulation time in vivo.²¹⁶ MOF can undergo in situ growth on the surface of Au NPs via coordination, encapsulating the Au NPs to shield them from the biological environment, thus enhancing their biocompatibility.²¹⁷ Zeng et al. modified the surface of Au NRs with SH-PEG, utilizing the coordination interaction between carboxyl groups and MOF to facilitate in situ MOF growth on the Au NRs surface. This method exploits the porous structure of MOF to achieve a high loading capacity of camptothecin (CPT). Cytotoxicity assays revealed that the cell survival rate after incubation with Au NRs@MOF reached 98%, confirming the biosafety of the material.²¹⁸ Similar to SH-PEG, PVP effectively prevents the aggregation of Au NPs through steric hindrance. Its high water solubility, low toxicity, and affordability have made it a widely utilized material in the biomedical field. Li et al. demonstrated that coating Au NPs with PVP masked certain surface-active sites and restricted the active regions available for interactions with analytes. This dual effect not only enhanced the particles’ anti-interference capabilities but also improved their biological stability.²¹⁹ Coating the surface of Au NPs with inorganic substances or modifying them with organic molecules enhances the overall biostability and biosafety of the material, thereby offering significant potential for clinical applications.

Au NPs have demonstrated promising results in experimental research for cancer diagnosis and treatment. However, several critical issues must be resolved before their application in clinical settings. Notably, the biosafety and biostability of Au NPs remain the most significant concerns. Although several hyperthermia-mediated nanomedicines for cancer treatment have been approved or are currently undergoing clinical trials, their clinical translation remains under investigation.²²⁰ Aurimune (CYT-6091) serves as a paradigmatic exemplar, which is the first SH-PEG coated and tumor necrosis factor-α (TNFα) conjugated AuNPs-based cancer therapy to reach early phase clinical trials (NCT00436410) and phase Ⅰ clinical trial (NCT00356980, NCT00436410). TNFα is a potent anticancer agent; however, its extreme side effects largely limit its clinical use.²²¹ In the study, the conjugation of TNFα to colloidal AuNPs is proposed to enhance the targeted delivery of TNFα to tumor sites, to mitigate the systemic toxicity of TNFα.²²² Considering the diagnostic potential of Au NPs, another clinical trial (NCT04907422) was conducted to identify highly sensitive biomarkers for the early detection and treatment of salivary gland cancer, aiming to improve patient prognosis and outcomes. The CD24 primer conjugated with Au NPs demonstrated efficacy in evaluating the prognosis of salivary gland tumors.²²³ 

In the current study, Zhu et al. employed Au NPs to target myeloid-derived suppressor cells (MDSCs) and the NLRP3 inflammasome within them to enhance the efficacy of PD-1 tumor immunotherapy. The findings suggest that inhibiting the NLRP3 inflammasome in MDSCs, while simultaneously enhancing the response of CD8⁺ T cells, may offer a promising strategy for cancer treatment.²²⁴ Lu et al. developed a nanodrug delivery system, M-Au@RGD-NM, which targets tumors and activates the STING pathway, significantly enhancing anticancer immune responses. This system not only promotes the maturation and proliferation of immune cells in both cell and animal models but also inhibits tumor growth in combination with RT by increasing the generation of ROS. Furthermore, the coating of neutrophil membranes further improves biocompatibility.²²⁵ Li et al., in their research on organic coordination compound-mediated supramolecular organic frameworks (MSOFs), have successfully developed a mask that achieves an iodine removal efficiency of 99.1% and exhibits real-time fluorescence. This advancement offers a novel solution to mitigate radiation exposure during RT.¹³ Lin et al. developed a nanosatellite drug delivery system, CAMD@CM, by encapsulating tumor cell membranes on covalent organic framework (COF) nanospheres. They loaded mannosylated Au NPs and DOX to enhance tumor-targeted photothermal chemotherapy and immunotherapy.²²⁶ Wei et al. synthesized Janus nanoparticles by combining Au NPs with Fe₃O₄ nanoparticles, and enhanced tumor targeting by functionalizing their surface with the activity-targeting ligand RGD (Arg-Gly-Asp). The researchers employed PTT to generate elevated temperatures, thereby promoting the Fenton or Fenton-like reactions for a synergistic therapeutic effect.²²⁷ Elechalawar et al. developed and assessed a specific Au NPs-based nanoformulation containing the anti-EGFR antibody cetuximab (C225) as a targeting agent and gemcitabine as a chemotherapeutic agent that effectively targets both PCCs and PSCs simultaneously. The study demonstrated efficient tumor accumulation of the nanoparticles, coupled with reduced uptake in essential organs, such as the liver, spleen, lungs, and pancreas, indicating minimal biotoxicity.¹²² In recent pioneering experimental studies, the majority of materials exhibited high levels of biosafety and biostability in both in vivo and in vitro experiments. These findings establish a robust foundation for the practical clinical application of Au NPs in biomedicine.

Au NPs represent a novel generation of nano-diagnostic platforms with the potential to transform early cancer diagnosis and precision therapy. Their physicochemical properties make them promising candidates for medical applications. The high-sensitivity biosensing technology enabled by the LSPR effect enhances the limit of tumor marker detection, while the passive targeting property, underpinned by the EPR effect, offers a novel approach for tumor tissue-specific enrichment. In terms of therapeutic applications, the multimodal synergistic therapy facilitated by Au NPs not only overcomes the limitations of conventional therapies in terms of selectivity but also enables spatiotemporal tumor ablation with high precision through its tunable light response. Studies have demonstrated that the multifunctional design of the integrated diagnostic and therapeutic platform can concurrently monitor the TME and perform targeted therapy, providing significant technological support for the advancement of cancer diagnostic and therapeutic paradigms.

It should be noted that the high cost of precious metal nanomaterials and complex procedures have impeded the practical implementation of SERS assays in clinical applications. In the complex environment of living organisms, factors such as temperature, pH, and ionic strength can affect the fluorescence signal intensity. The presence of other biomolecules in the solution can interfere with the color change, resulting in unstable detection results and even false-positive results. Further research and improvement are necessary to enhance the performance and application value of fluorescence and colorimetric sensors. In the context of cancer diagnosis, while research on biosensors based on Au NPs has achieved notable advances, inherent defects persist, underscoring the need for continued improvement and refinement.

Conventional therapeutic modalities currently remain predominant in cancer treatment, while Au NPs-mediated PTT and PDT, along with similar approaches, represent more advanced and innovative strategies. However, these novel therapies are still in clinical trial phases and have not been widely adopted owing to potential risks of adverse reactions, inherent technological limitations, and the physiological characteristics of the TME, which also restrict therapeutic efficacy. Common PTAs and PSs often exhibit low targeting precision and efficiency, hindering effective tumor accumulation. In addition, the efficacy of monotherapy is frequently modest, and in cases of limited excretion, potential renal toxicity may arise. The combination of PTT and PDT using Au NPs has demonstrated enhanced therapeutic effects through a synergistic mechanism. Nonetheless, the hypoxic environment within tumor tissues can impede the generation of ROS by PDT, and the need for distinct NIR wavelengths for both therapies can reduce their overall efficacy. Future research should focus on three primary directions. First, by modulating the core–shell structure and engineering surface ligands, the development of a multifunctional Au NPs system with intelligent response characteristics is essential to achieve “on-demand diagnosis and treatment” through TME-specific activation. Second, advancing the integration of multimodal diagnostic and therapeutic technologies is critical, particularly through the cross-fertilization of optical therapy with immunomodulation, gene delivery, and other approaches. This would lead to the creation of an integrated “detection, treatment, and prognosis monitoring” system. Third, the implementation of a standardized biosafety evaluation system is necessary to systematically investigate the long-term in vivo metabolic mechanisms and potential toxicity, which would support the clinical translation process. The integration of artificial intelligence-based nanoparticle design strategies and 3D tumor model verification platforms holds great promise for overcoming the limitations of the traditional trial-and-error model in research and development. The synergy between precision medicine and nanotechnology is expected to drive a paradigm shift in personalized cancer treatment, ultimately achieving the clinical goals of “early diagnosis and precise treatment.”

This work was supported by the State Key Laboratory of Pathogenesis Prevention and Treatment of High Incidence Disease and Central Asia, Xinjiang Medical University (Grant No. SKL-HIDCA-2023-15), the Natural Science Foundation of Xinjiang Uygur Autonomous Region (Grant No. 2022D01C727), the Talent Project of Tian chi Doctoral Program in Xinjiang Uygur Autonomous Region (Grant No. 0301050903), the Special Funds for Talents of Xinjiang Medical University (Grant No. 0103010211), the Key Scientific Research Program of Shaanxi Provincial Education Department (Grant No. 23JS016), the Natural Science Foundation of Xinjiang Uygur Autonomous Region (Grant No. 2022D01C715), and the College Students’ Innovative Entrepreneurial Training Plan Program (Grant No. X35).

The authors have no conflicts to disclose.

B.L. and M.Y. equally contributed as co-first authors. All authors have read and agreed to the published version of the manuscript.

Bingrui Li: Conceptualization (equal); Project administration (supporting); Writing – original draft (lead). Maihemuti Yakufu: Conceptualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Ru Xie: Visualization (lead); Writing – review & editing (supporting). Hanfei Peng: Writing – original draft (supporting). Xiaohu Mi: Funding acquisition (equal); Project administration (equal); Writing – review & editing (equal). Hairegu Tuxun: Funding acquisition (lead); Project administration (equal); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.