Enhanced adsorption and non-linear optical properties of DNA-CTAB functionalized mesoporous silica nanoparticles and their influence on enhancement of photoluminescence of Rhodamine 6G dye

The emergence of silica nanoparticles has been brought to attention in material chemistry since the 1970s, soon after the development of nanotechnology. In general, silica is defined as transparent, odorless, vitreous luster, non-conductive to electricity, diamagnetic, and amorphous. These properties of silica limit their roles in certain applications, such as a substrate for thin film making, fiber optics, ceramics, and semiconductors. Since the invention of the Stober process in 1968, it is proven that nano-level, controllable spherical silica particles can be achieved. Subsequently, various synthesis methods were reported to obtain controllable and uniform-sized silica nanoparticles in material science for different industrial and chemical aspects. Moreover, the material research field realized that silica nanoparticles were undergone surface modification under different experimental conditions and acquired adsorptive and non-linear optical properties. The morphologically modified silica particles are specified as mesoporous nanoparticles in which the pore size falls to 1–10 nm. In addition, they offer a porous structure, tunable size, large surface area, biocompatibility, and ordered uniform structure.^1–18,20 This awakened the material chemistry in a way that silica can be subject to various usages due to its enhanced morphological adaptations and non-linear optical features. Over the last few decades, various chemical and physical methods were introduced to enhance the properties of silica nanoparticles at different morphological and functional levels. Based on the easiness of production and physio-chemical features, nowadays silica nanoparticles became one of the class materials, which are known for their different roles in the chemical industry, such as nanomedicine, catalysis, plasmonic color thin films, surface-enhanced spectroscopy, deoxyribonucleic acid (DNA) extraction, magnetic separation, and photonic devices and instrumentation development. Recently, porous nanomaterials are widely used for tuning laser systems and cell imaging purposes. Among the porous nanomaterials, silica nanoparticles are treated as a favorite subject in material science, so silica-incorporated research works become a focus in tuning dye laser systems.¹⁹ These applications of silica are merely based on its unique mesoporous surface properties. The surface properties of silica nanoparticles can be modified for different purposes. These features of the mesoporous silica nanoparticles help to attach the functional chemical groups on them in comparison with amorphous colloidal silica. The reported research papers revealed that mesoporous silica nanoparticles can influence the structural, microstructural, and biological properties of polymer nanocomposites. Functionalized mesoporous silica nanoparticles are widely recognized for their stable chemical properties, controlled bioactive compound release, adsorbent-adsorbate stability, thermal stability, control of hydrophilicity, and pH resistance. The surface properties of silica nanoparticles can be enhanced by altering various parameters, such as temperature, pressure, the molar ratio of reagents, pH, and chemical additives as well. The presence of chemical additives during the precipitation of silica nanoparticles can control the growth of nanoparticles within the mesoporous limit along with the surface modification of other parameters. Since the features of silica nanoparticles in terms of porosity, size of the pores, pore volume, surface area, and biocompatibility are found to enhance by the presence of chemical additives, different methods and chemical additives have been reported.^1–18,20 In 2009, Qiao et al.²¹ reported the synthesis of mesoporous silica nanoparticles through a controlled hydrolysis process using cetyl trimethyl ammonium chloride (CTAC). In this article, they claimed that they could control the size of the silica nanoparticles to a range of 25 to 200 nm. In 2012, Lodha et al.²² stated that they have synthesized mesoporous silica nanoparticles for drug loading of the purely water-soluble drug. They used cetyl trimethyl ammonium bromide (CTAB) and concentrated hydrochloric acid (HCl) for the synthesis of mesoporous silica nanoparticles. In 2017, Vazquez et al.²³ claimed that they could achieve to form silica particles having pore volume sizes of 2.5 to 2.8 nm. They have used CTAB as the pore-generating agent. In 2020, Presentato et al.²⁴ stated that they prepared biodegradable, high-pore volume mesoporous silica nanoparticles using CTAB and concentrated HCl. These reported silica particle synthesis methods were carried out using the Stober process method. Such mesoporous silica nanoparticles are applied to the biomedical field for target drug delivery, bio-sensing, and cellular uptake. Since their role in the bio-medical field is found to increase, the recent works mainly focus on making mesoporous silica to be functionalized and biocompatible using different surfactants, such as cetyl trimethyl ammonium chloride (CTAC).^1–32 

This work introduces a biotemplate to prepare mesoporous silica nanoparticles by incorporating the Stober process. Among the available reported synthesis methods of silica nanoparticles, we have approached an effective bio-template to obtain tailored morphology of silica particles. The bio-template was synthesized by forming a complex of deoxyribonucleic acid and the cationic surfactant cetyltrimethyl ammonium bromide (CTAB). The crucial roles surfactants play in nanoparticle formation are described by adsorbing the nanoparticles to their surface and lowering the surface energy of the nanoparticles soon after they are formed. This is the way in which the aggregation of particles has been prevented and the formation of size and surface controlled nanoparticles. So far, the stabilizing mechanism of DNA in nanoparticle formation has been recognized, as its usage is being increased in material science as a bio-template and conjugate polymer as well. Studies show that the nanoparticles that are formed in the presence of a DNA template exhibit excellent physiochemical properties.^33–61  Figure 1 illustrates a schematic representation of the polynucleotide structure of DNA. Since the DNA is considered as a fascinating biophotonic material, it is verified in many research papers that it can improve the functional properties of photonic device materials. Moreover, the seductive factor that makes DNA to be a vital material in material science is the double helical-lengthy polymer structure of the molecule. Due to the unique double helical structure, DNA possesses high transparency, thermal stability, non-linear optical activity, and amplified emission, its applications are widely found in electron blocking, hole transport, hosting of laser dyes, modification capability, electrical interconnectors, thin films, lasing, and sensing purposes. The strands of the double helical structure have been made of alternating sugar (deoxy ribose) and phosphate groups. The sugar group is attached to one of the four bases: adenine (A), cytosine (c), guanine (G), or thymine (T). The stability of the dsDNA molecule depends on GC base pairs and length. It is found that longer molecules are more stable. Long DNA helices with a high GC-base pairs have more strongly interacting strands, while short helices with high AT content have more weakly interacting strands. The structural features that defines a DNA polymer to engage for a reaction are length, base sequence, temperature, and buffer condition.^33–60 

As far as concerning the diverse properties of DNA, it is found that they can be incorporated with nanoparticles, which have positive charge. Mostly, DNA is used for functionalizing with metal nanoparticles to form functionalized or conjugated polymer under a buffer condition. This confirmed the negative charge of the DNA that resembles the chemical behavior of ligand. The studies dealt with its conjugating effect as they are taking part in a chemical reaction with other polymers (surfactants, metal nanoparticles, etc.) they got functionalized, so that they can form organic–inorganic complexes and salt with the polymers. In turn, the functionalized DNA has diverse properties that it can tune nanomaterials about their size, shape, surface compatibility with adsorbents, electrical, magnetic, optical, and biological properties.^56–61 

The critical disadvantage of the DNA polymer, which prevents it from using directly in the functionalization of silica, is that the solvent used to synthesis silica nanoparticles in the Stober process is an alcohol medium (either ethanol or methanol). As DNA alone cannot readily dissolve in alcoholic solvents, it is required to transform DNA into its complex form with a cationic surfactant. Considering the properties of the functionalized DNA polymer, we selected cetyl trimethyl ammonium bromide (CTAB). It is also used for synthesizing mesoporous silica nanoparticles. It is a known cationic surfactant and belongs to one of the compounds of the cetrimonium cation family. The fascinating feature of CTAB is that it is widely used as the main component of the buffer solution for the extraction of DNA so that it reacts with the polysaccharide chains of the DNA.^{16,19,61–63}

Its chemical formula is hexadecyltrimethyl ammonium bromide ([C₁₆H₃₃)N(CH₃)₃]Br) and is highly biocompatible. Figure 2 is a schematic representation of the CTAB structure. It is a familiar compound to be used for nanoparticle synthesis. It has 16 carbon as a long tail and an ammonium head group with three methyl groups attached. In some cases, functionalized biomolecules, such as DNA, cannot sufficiently penetrate a dense layer of capping agents or replace them. Therefore, surfactant-assisted methods aim about modifying the surface ligands by adding excess of small molecules with higher affinity toward dielectric nanoparticles. In the case of the DNA molecule, the deoxyribose-phosphate groups contribute to the backbone of the DNA strand. Among these groups, the phosphate group comprises one negatively charged oxygen atom and is responsible for the negative charge of the entire DNA strand. It is the result of the presence of bonds between phosphorous and oxygen. Hence, the cationic surfactant becomes attached to the phosphate group of the DNA and forms the DNA–surfactant complex.^{20–22,30–32}

Meeting the features of CTAB and DNA together, we proposed CTAB as the cationic surfactant for synthesizing the CD complex without using any buffer chemical. Silica is a well-known substrate material, and vast synthesis methods of mesoporous silica are available. Majority of the applications of mesoporous silica are broadly spanned in medical purposes, especially in the drug delivery system, binding of biomolecule for extraction of cell organelles, and cell imaging purposes. Therefore, functionally modified silica should be biocompatible and has to possess tailored morphological properties. Keeping this fact, the relevance of this work among other synthesis methods of silica is that this method proposes very simple, cost effective, and functionally modified silica with the DNA polymer without using any buffer solutions. By excluding buffer solutions, we can avoid toxic chemicals during the synthesis process and make the experimental conditions free from the toxic environment. It is found that the CD complex can dissolve in alcohol group. The obtained mesoporous silica nanoparticles have shown an enhanced tailored morphology along with the controllable adhesive property. It is also proven that the CD bio-complex template was an effective template to incorporate with the Stober process to enhance the particle size, pore size, volume, and area of the silica particles. The fluorescence property of Rhodamine 6G (R6G) dye particles was studied by encapsulating them with obtained silica nanoparticles in dimethyl sulfoxide (DMSO). The study shows that the fluorescence of dye particles was enhanced with the presence of CD-functionalized silica nanoparticles. The non-linear optical properties of dielectric materials with metal composites were investigated since two decades ago for fabricating non-linear optical devices. In those studies, the non-linear optical properties of metal nanoparticles doped with silica glass substrate were only considered.⁶⁴ This pointed that the non-linear optical property of mesoporous silica nanoparticles alone is seemed ignorant in non-linear optical physics.⁶⁵ Even though, silica glass inclusion with semiconducting materials and metal nanoparticles in order to enhance the non-linear optical properties of the doping materials is still maintained as a research interest. DNA is a well-known non-linear optical material, and the nanoparticles, which are functionalized with DNA, also possess nonlinear optical properties. Hence, along with the morphological analysis, we have also verified the non-linear optical properties of the CD-functionalized silica nanoparticles using the Z-scan technique.

Chemicals used were CTAB, DNA, tetra ethyl orthosilicate (TEOS), ammonium hydroxide (NH4OH), methanol, hydrochloric acid (HCL), acetone and deionized (DI) water. All chemicals except DNA were purchased from Merck Life Science Private Limited. DNA powder was purchased from Sisco Research Private Limited. All the glass wares used for the experiment were cleansed by acetone, followed by DI water, dried in hot air oven at 100 °C, and were again subject to Piranha cleaning followed by DI water cleansing and dried at 100 °C using hot air oven.

We prepared the complex of DNA and the cationic surfactant CTAB with different concentration of DNA solution and kept the concentration of CTAB constant. The concentration of DNA solutions were taken as 0.1, 0.5, 0.15, and 0.1525 of weight-percentage (wt. %), and they were prepared by dissolving DNA in DI water followed by sonication at room temperature for 30 min to achieve homogeneous solution. In another beaker, 0.4 g of CTAB was taken, and 30 ml of DI-water was poured into the beaker and sonicated until the CTAB was completely dissolved at room temperature. It was a colorless viscous solution. The CTAB solution was added slowly into the DNA solution under constant stirring using sonication at room temperature. This process allowed to precipitate a white colloidal form of a CD complex where the surfactant CTAB attached with the DNA molecule. The precipitation was filtered and washed with DI water in many times to remove unreacted CTAB. The CD complex precipitate was kept in a hot air oven at 60 °C for 24 h to obtain CD complex powder.^48,49 Figure 3 shows a schematic representation of CD complex formation.

The silica nanoparticles were synthesized in the presence of the CD complex under the conditions of the Stober process.¹ The samples of 0.01, 05, 0.15, and 0.1525 wt. % of CD complex powder were dissolved in each of 150 ml of methanol under constant stirring. 35 ml of DI water was added into each four CD complex-methanol solutions under constant stirring for 10 min at room temperature. 5 ml of tetraethyl orthosilicate solution was added into these solutions under constant stirring using ultra sonication at room temperature in a 500 ml borosilicate glass beaker. 14 ml of ammonium hydroxide was added as catalyst slowly into the solution under constant stirring. The solution was kept stirring for 30 min at room temperature, and a white colloidal precipitation was formed during stirring. The colloidal form was kept for one hour for complete gelation. The precipitation is filtered by waterman filter paper and washed 5 times with DI water followed by methanol. The filtered precipitation was kept in a hot air oven at 70 °C for 1 h for drying. Then, the obtained silica powder was calcined at 10 °C/min. In addition, we have performed different experimental conditions for observing the effect of CD complex on making silica nanoparticles. Figure 4 shows the different methods involved in the formation of silica nanoparticles. One of them was carried out without the CD complex. In order to distinguish the roles of DNA and CTAB individually, one sample of silica nanoparticles was synthesized using CTAB as the surfactant.¹⁹ The Stober process can be performed in both alkaline and acidic medium. Therefore, the reliability of the CD complex were examined in both acidic and alkaline conditions of the Stober process. For acidic condition, we have used concentrated HCl. During the preparation of silica nanoparticles using the Stober process, 5 ml of concentrated HCl was added immediately after the colloidal form of silica gel is formed. This method is called the peptization process, and it is generally used for making mesoporous silica nanoparticles. Figure 5 illustrates a schematic representation of peptization method, which is performed in both the absence and presence of the CD complex.

Figure 6 shows the difference between the FWHM of S1 and S2 due to the presence of CD complex template. When comparing the XRD patterns of S1 and S2, the diffraction peak of S1 tends to move lower angle and getting narrower than the peak of S2. These features of S1 indicate that the silica particles may have crystalline structure or better orientation.³⁵ From the FWHM difference, it is understood that the size of the particles of S1 is smaller than as that of S2. The XRD results validate that the presence of CD complex template influences the size of the silica particles to reduce at the nano-scale range.

The optical absorption spectrum of the samples were carried out using UV–vis spectrophotoscopy in the scan range of 230–800 nm wavelength. The silica samples subjected for UV–vis analysis were S1, S2, S3, S4, S5, S6, S7, and S8 corresponding to silica prepared by the Stober process, the Stober process using CTAB as the stabilizing agent, 0.01, 0.05, 0.15, and 0.1525 wt. % of DNA concentrations of the CD complex in the Stober process, and the peptization process with 0.1525 wt. % of the CD complex template, respectively. The amount of CTAB used for synthesizing sample S2 is approximately the same with the amount of CTAB used for preparing samples S7 and S8. This experimental condition is maintained for differentiating the background role of CTAB in the CTAB incorporated Stober process, CD complex template involving Stober and peptization processes for forming the nanoparticles. The solvent used for UV–vis analysis was dimethyl sulphoxide (DMSO). Figure 7 represents the UV absorption spectra of silica samples. It illustrates that the absorption peaks for silica prepared in the CD complex are showing blue shift (samples S2–S8). The peak blue shift for S8 is approximately about 10 nm. This indicated that as increasing the concentration of the CD complex template, the particle size goes on decreasing. This result exemplifies that the presence of CD complex reduces the size of the silica nanoparticles.

The peak shift difference between S3 and S8 indicates that the CD complex can also make impact in acidic condition so that S3 and S8 are prepared under acidic medium.

Surface area is the one of the quantities to explain the particle size, particle morphology, surface texturing, and porosity of any particle. It is considered as the one of the vital factors, which explains the potential of physical properties of particles on taking part in biological and inorganic catalytic activities. To endorse the surface area of the silica nanoparticles derived from the CD complex templated method, BET analysis was conducted. Based on the BET analysis results, N2 adsorption/desorption isotherms were employed to identify surface parameters. We have subject six samples of silica particles for BET analysis. The samples were named as S1, S2, S3, S4, S5, and S6 corresponding to the silica particles of prepared by the Stober process, prepared by using 0.01, 0.05, 0.15, and 0.1525 wt. % in the peptization method and 0.1525 wt. % CD complex template. The BET results discussed the size of the pores, the type of pores whether it is open or closed, and the pore diameter distribution. The N2 adsorption/desorption isotherm curves of the six samples of SNP exhibit type IV hysteresis loop with two branches, adsorption (capillary condensation) and desorption (evaporation). These hysteresis loops are indicating the presence of open pores between the particles and mesoporous surface. Since the Stober process gives mesoporous silica nanoparticles, this general feature is supported from the hysteresis loops of the all the six samples.⁷⁴ 

The adsorption branch of type IV is composite of type I and II. At low relative pressure (p/p₀), the uptake of gas molecules is associated with the filling of micropores (forms monolayer adsorption). These type IV loops are often found with aggregated crystals of zeolites, some mesoporous zeolites, and micro-mesoporous carbon. The isotherm curves of samples shown in Fig. 8 illustrated that the adsorption pore volume of the samples went increasing as going from sample S1 to S6 in accordance with the pore diameter, which shows a gradual increase on increasing the concentration of the CD complex template.

The highest value of BET surface area is for sample S6. It is evident from the isotherm data shown in Table I of the samples that, for large pore size, the BET surface area is comparably found to low, which implicates that for low mesoporous properties, and such a surface cannot adsorb particle to their surface than particles having small pores on their surface. For small pores, the BET surface area and pore volume are large. This feature implies that their adsorption stability is high. This is evident from the isotherm curves of samples S1–S6 shown in Fig. 8. At higher relative pressure (p/p₀), the hysteris loop width is quiet wider for samples S5 and S6 as comparing with other samples. The fact that connects the loop width and adsorption property is that the wider the loop, the adsorption stability is also higher. Moreover, the hysteresis loop width is found to increase as going from S1 to S6. The only factor that varies as going from S1 to S6 is the concentration of the CD complex template. Samples S5 and S6 were made in the presence of high concentration of the CD complex template (0.1525 wt. %) among other samples. Another fact obtained from the BET results is that the pore size of all the samples are within the mesoporous range (2 nm < pore diameter< 50 nm). Among the six samples, S6 exhibited a small value of mesoporous size by analyzing the size distribution of the pores. In fact, this pointed that the presence of CD complex induces to make the porous surface of the silica nanoparticles to become more adsorbent surface in which the adsorbents cannot be escaped easily. As we know, DNA is a double stranded lengthy biopolymer and CTAB is also a lengthy polymer. These together form a complex structure that is spanned between the silica particles. Due to the lengthy network of the CD complex, the silica particles cannot be aggregated beyond a particular range, and thus, nanoparticles are formed. The nanoparticles are confined in the CD complex surface, after the removal of the CD complex, which leads voids in the surface of the silica nanoparticles, which, in turn, caused the silica particles become mesoporous. This feature makes them to be a good adsorbent and that can be efficiently used for drug delivery agent, doping agents, and gas sensing applications.

The SEM images of the silica particles are shown in Fig. 9. The silica particles prepared without the CD complex template is seen in Fig. 9(a). The particles are showing a spherical geometry with smooth surface. Figure 9(b) is representing the silica particles prepared in the presence of CD complex, in which the particles are seen in aggregated spherical and flat surfaces but irregular size. The surfaces of the particles were seemed to be irregular spherical structures, which were seen as projecting from the surface. These projected like spherical nanoparticles can enhance the surface area. Hence, the SEM results show that the presence of CD complex influences the morphology of the silica particles to a favorable feature of mesoporous surface, and this synthesis method could preserve the spherical shape of the silica particles, which is seen as a general feature of the Stober process.

DNA, the major component of the biotemplate CD complex, is a well-known non-linear optical material.^{34–37,42,55,58–61} It is proved that non-linear material can influence non-linear optical properties of another material when functionalizing each other.^42,58–61 In order to understand whether any change has been formed in the non-linear optical property of the silica particles, we have conducted open aperture Z scan experiment. The open aperture Z-scan analysis was carried out under 100 µJ laser excitation (532 nm, 9 ns pulse rate) with an on-axis intensity of 2.46 GW cm⁻². The transmitted beam was measured without aperture in front of the detector to determine the non-linear absorption of the molecules. The linear transmittance was set at 71%. Figures 10(a), 10(c), 10(e), 10(g), and 10(i) show the open aperture Z-scan signatures of silica particles prepared at different concentrations of the CD complex template in different methods. The silica particles was uniformly dispersed in DMSO for carrying out open aperture Z scan analysis. The interaction of the laser beam with the molecules produces a valley pattern, i.e., the transmittance of the molecules decreases gradually toward the focal point and reaches a minimum with the deep transmittance trough at the focus where the curves are symmetric at Z = 0, which signifies the reverse saturation absorption (RSA) behavior of the molecules with the positive NLA of the incident light. On the nanosecond time scale, the RSA is combined with two photon absorption (TPA) and excited state absorption (ESA), which is collectively as the effective TPA process.^61,75–78 The experimental data are matching well with the theoretical model for the ESA assisted TPA process in all six samples of silica nanoparticles.

The numerically fitted OA Z-scan results brought to light the enhanced NLO responses of the mesoporous silica nanoparticles synthesized at different concentrations of the CD complex template in the Stober process method and peptization method. The obtained β_eff were tabulated in Table II.

The optical limiting data of silica samples for different concentration of DNA of CD complex template were extracted from the graphs of normalized transmittance obtained from open aperture Z-scan analysis against input fluence [Figs. 10(b), 10(d), 10(f), 10(h), and 10(j)]. From the graph, the onsets of limiting action (the value of input fluence at which the intensity of output transmittance starts decreasing) are observed at 3.29, 4.74, 5.23, 8.97, 1.46, and 2.16 W/m² for the corresponding silica particles synthesized at 0.05 wt. % of the CD complex template, 0.15 wt. % of the CD complex template, 0.1525 wt. % of the CD complex template, peptization method, and 0.1525 wt. % of the CD complex in the peptization method, respectively, and the limiting threshold (LT) values (the value of input fluence at which the intensity of output pulse becomes 50% of the initial value) for the corresponding samples are 2.43, 2.35, 2.21, 1.24, and 0.75 of the order of 10¹³ W/m² respectively. The values of the optical limiting threshold corresponding to each sample was tabulated in Table II. From the results, it is evident that the particles in the present study exhibit exceptional optical limiting action with low onset and limiting threshold values. The open aperture Z scan report shown that the sample S5 shows great non-linear absorption coefficient and has low optical limiting threshold value. The studies on mesoporous nanoparticles show that the non-linear optical property can be enhanced by reducing pore size and particle crystalline orientation.⁷⁷ It is noticed that the enhancement of nonlinearity may be due to the influence of the CD complex template during the formation of silica particles. However, the materials in the present study have shown remarkable nonlinear response with substantial increment in β_eff of the order 10⁻¹⁰ m W⁻¹ and exceptionally well optical limiting behavior with very high limiting threshold, making them capable materials for optical power limiting devices in photonics.

The influence of porous substrate on the enhancement of photoluminescence of dyes is always a subject of laser applications.²⁸ We have selected Rhodamine 6G (R6G) as its inclusion with various nanomaterials, such as, silica has been reported to improve the photoluminescence property. Moreover, it is a known cationic dye and has a strong optical absorption with high fluorescence yield. As already discussed in the Introduction, silica possesses a negative polarity in aqueous medium; therefore, R6G has an affinity to adsorb on the silica surface.

We have conducted UV–visible absorption spectra of R6G– DMSO solution with different concentrations of CD complex templated silica nanoparticles. We have taken three R6G–DMSO solutions of samples A, B, and C, of which samples B and C were dispersed with 0.1 and 0.3 g of silica nanoparticles, respectively. From Fig. 11, the UV absorption peak was shifted to the red region on increasing the concentration of silica particles. The UV–visible absorption peak of R6G in DMSO (sample A) was found around at 520 nm. The corresponding absorption peaks of B and C are seen around at 535 and 539 nm, respectively. This red shift of the absorption peaks indicates that there is a change in the electronic ground state and excited state of the R6G molecules so that the size of the energy gap between the dye particles changes as the nature of solvent (DMSO) changes. As on adding the silica particles to DMSO, a dispersive force (interaction) between DMSO and silica particles would take place. This dispersive force is influenced by the polarity of both solvent and solute. As we know, silica is a highly polar compound and DMSO is a polar solvent. For a polar solute in polar solvent, dipole–dipole interactions will stabilize the ground state of the solute according to the polarity of the solvent. In this case, the solvent molecules will be oriented around the ground-state dipole, and if the dipole moment is increased by the transition, the excited state will be stabilized more by more polar solvents (those with higher dielectric constants). This reflected in the absorption or emission spectrum of the solute as differences in the position, intensity, and shape of the spectroscopic bands. This effect is called solvatochromism.^79–82 When the spectroscopic band occurs in the visible part of the spectrum, solvatochromism is observed as a change of the color. From this observation, it is clear that on the addition of silica particles to the DMSO, its polarity got changed and, subsequently, the orientation of the DMSO molecules. Therefore, when considering the polarity and orientation of DMSO of samples B and C, they have been changed as comparing with sample A. Therefore, the excitation energy of the dye molecules is decreased, thereby a red shift occurred. Moreover, the particles present in the solvent influence the polarity of the solvent, and the solvent–solute bonding may increase. Thereby, Fig. 11 provided an additional information about the stability of solubility of the CD complex templated silica nanoparticles in DMSO.

Additionally, Fig. 11 also shows that the absorption peak value is seen as reducing on increasing the CD complex templated silica particles. This may be due to that as the inclusion of CD complex templated silica in the DMSO, they were uniformly dispersed in the solvent and formed as a platform to adsorb R6G to their surface. Hence, the number of dye particles in the solvent along the path of the UV–visible beam was reduced; hence, this, in turn, affected the absorption peak value.

Photoluminescence (PL) response of R6G in different polar state of DMSO was conducted using the OHSP-350 model of the spectral analyzer. The experiment was carried out in eight differently prepared silica samples dispersed DMSO to analyze the PL characterization. Sample S1 is corresponding to silica nanoparticles prepared using the Stober process without any surfactant; S2 is for silica particles prepared in the presence of CTAB as the surfactant in the Stober process; S3 belongs to silica particles prepared by the peptization method; and S4, S5, S6, S7, and S8 are corresponding to the silica particles, which were prepared in 0.01 weight percentage of the CD complex, 0.05 weight percentage of the CD complex, 0.15 weight percentage of the CD complex, 0.1525 weight percentage of the CD complex, and 0.1525 weight percentage of the CD complex in the peptization method, respectively. The weight of silica for eight samples was maintained with the same quantity, and it was 0.0124 g. The eight samples of silica were added in 20 ml of DMSO and sonicated at room temperature until the silica particles were dispersed uniformly. Another solution of 0.01 mM of R6G in DMSO was prepared. From Fig. 12, it is clear that the PL response of R6G shows an enhancement as going from silica samples S1 to S8. Figure 12 pointed that the PL response was so high for samples S4–S8, which were synthesized using as the order of increasing concentration of the CD complex template. As the concentration of the CD complex template increases, the silica samples also show improved physical property, which was also discussed in the BET analysis. The CD complex templated silica nanoparticles provided large area to adsorb R6G particles on their surface. As going from S4–S8, the adsorption rate is also increased. The silica particles in DMSO were uniformly dispersed and their stability to hold with the solvent molecule is so high so that their duration to present in the DMSO is also high. These facts, in turn, enhance the absorption of spectral energy against the R6G particles so that the concentration of dye molecules is decreased in the solvent, and thereby, energy could distribute through the medium in a way that dye molecule could get enough time to absorb sufficient energy. In addition, the silica particles encapsulated the dye particles; hence, this may reduce the oxidation of dye particles and make the dye particle to become photostable. The presence of silica nanosubstrate in the solvent also reduced the interaction between dye molecules. Thereby, the concentration quenching can be avoided. These all features may be the net effect in the enhancement of PL response of R6G. This enhancement of photoluminescence of R6G can be used to lower the lasing threshold of gain medium in the tunable dye laser systems and also can be used for synthesizing photo stable dye incorporated silica nanoparticles for cell imaging purposes.^83–85 

Mesoporous silica nanoparticles were synthesized using the Stober process and peptization method in which surface modified DNA with CTAB was used as the capping agent. The characterization results show that the CD complex provides twin benefits of both DNA and CTAB during the synthesis procedure. The CD complex acted as a stabilizer, which enhanced the morphological and the non-linear optical properties of the obtained silica nanoparticles without altering the spherical geometry of the silica nanoparticles. The XRD and UV results confirmed the formation of silica nanoparticles, and they also show the presence of CD complex functionalized silica. In XRD results, the FWHM was higher for the silica nanoparticles, which were prepared in the presence of CD complex. The UV results also proved that the concentration of the DNA effects the size and porosity of the silica particles. The particles size were reduced by the effect of DNA, and it was confirmed by varying the concentration of DNA. As the concentration of the DNA was increased, a blue shift in the peak is observed. This indicated that the presence of DNA in the synthesis process influenced in reducing the size of the silica particles. The surface properties of the obtained silica particles were studied by BET–BJH analysis. BET results show that on increasing the CD complex concentration, the pore diameter was found to decrease within the mesoporous range. The silica nanoparticles obtained from the CD complex template have remarkable hysteresis loop. This property of them helped to stick the adsorbent tightly to the adsorbate. The SEM characterization revealed the surface modification occurred in the silica nanoparticles formed in the presence of CD complex template. In addition to this, the SEM results shown that the presence of CD complex did not alter the specificity of the Stober process. Non-linear optical characterization was also performed by Z-scan techniques. With that shown, that the novel silica nanoparticles show a significant improvement of the non-linear optical absorption property, and their nonlinear absorption coefficient is found to increase on increasing the concentration of CD complex template, and the optical limiting threshold value of the CD complex templated silica nanoparticles was also reduced on increasing the concentration of DNA.

All the characterization results pointed that the CD complex works effectively as a stabilizing agent encompassed with the conventional Stober process for synthesizing mesoporous nano-sized non-linear optical silica nanoparticles. This method can be performed in both acidic [peptization method (HCL)] and basic media (the Stober process in which ammonia is used as the catalyst). The effect of CD complex templated silica nanoparticles is also found in the enhancement of photoluminescence of R6G. This peculiarity of silica nanoparticles will be a promise for the tunable dye laser system. While using the CD complex as the biotemplate, the toxic reagents and the complicity of experimental procedure can be avoided during the synthesis of mesoporous silica nanoparticles. Hence, the CD complex incorporating the Stober process is a novel, non-toxic, and cost effective method for synthesizing surface modified, tunable structured, biocompatible, and enhanced non-linear optical silica nanoparticles. These features of the silica nanoparticles make them to be used for drug delivery, catalysts, surface modified thin films using the doped surface, lasing, bio-sensing, gas sensing, bio-imaging, and semiconducting-photonic devices.

The authors conveyed their gratefulness to the University of Calicut; NIT Calicut; Bharathidasan University, Thiruchirappalli; and Government College for Women, Thiruvananthapuram, for the different studies of synthesized samples.

The authors have no conflicts to disclose.

All authors contributed to the study conception and design. Material conception, experimental design, data collection, and analysis were performed by Bhagyasree G. S., Reena V. N., Abith M., Sabari Girisun T. C., and Nithyaja B. The first draft of the manuscript was written by Bhagyasree G. S., and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

G. S. Bhagyasree: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Resources (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). V. N. Reena: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing – review & editing (equal). M. Abith: Conceptualization (equal); Methodology (equal); Resources (equal); Software (equal); Writing – review & editing (equal). T. C. Sabari Girisun: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Resources (equal); Software (equal); Writing – review & editing (equal). B. Nithyaja: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.