Plasmonic enhanced fluorescence via 3D printing spiral conical tapered gold tip bound to optical fiber

Fluorescence analysis has been a dominant technique in biochemical sensing and medical diagnostics studies due to its high sensitivity, great simplicity, and broad availability of fluorophores. Despite the great progress made in the past decades, there is still a drive to further improve these three factors so that fluorescence analysis can be used in an increasing number of scenarios with higher sensitivity and better precision. The detection limits or the fluorophores’ detectability is eventually limited by their quantum yield, photostability, and autofluorescence.¹ In the last decade, plasmonic enhanced fluorescence (PEF) has attracted significant attention as an emerging tool for better fluorescence spectroscopy, as this approach can greatly enhance fluorescence emission, decrease fluorescence lifetime, and, thereby, improve fluorophore detectability.² In fact, the excited surface plasmon provides a significant enhancement of optical field intensities and consequent light–matter interaction strengths in the vicinity of metal nanostructures, and these highly enhanced local fields enrich optical spectroscopy with new capabilities for photon-driven processes. The observed fluorescence augmented spectral intensity in many experiments can, thus, be credited to the local field enhancement associated with the excitation of localized surface plasmon resonances (SPR) in the metal nanostructures and resultant increased molecule absorption and emission cross sections.^3,4

The realization of a substantial PEF effect generally requires the preparation and fabrication of noble metal substrates, such as Ag, Au, and Cu,^5–10 with roughened surfaces or patterned nanostructures. Yet, the effective enhancement distance, as determined by the high localization feature of SPR, is typically less than 100 nm, and this has severely limited the practical application of PEF, especially in biosensors.^11,12 Cells need to be transferred and live on the metal substrate which may alter the behavior of cells in their original environment. In addition, measurements of such substrates are traditionally performed using attenuated total reflection (ATR) in a prism, also known as the Kretschmann configuration, employing a prism for resonant coupling of laser photons from the backside of the prism through the thin noble metal layer to surface-plasmon waves at metal–solution interfaces.^13,14 Although this approach offers high spectral resolution, high sensitivity, and simple experimental realization, it is somewhat cumbersome, expensive, and not applicable for remote sensing. To overcome these weak issues, waveguides and fiber based PEF configurations that offer advantages of easy coupling, miniaturization, and potential for multiplexing have been proposed.^15–17 

An optical fiber can become a promising platform for remote monitoring when in combination with PEF. Thanks to the capability of guiding light with low losses, intrinsic flexibility, and immunity to any electromagnetic interferences, optical fibers represent a key element in many technological fields from optical communications to the development of robust, reliable, and high-performance diagnostic systems and optical sensors.^18–21 In many cases, there have been great interest in implementing various dimensional/shape modifications on the far end of the fiber based on the excellent remote transmission characteristics of general fibers. A typical example is to prepare a tapered tip of the optical fiber. As the light beam inside the fiber approaches the very end of the tip, the evanescent field component becomes broader and broader, extending toward the surrounding medium. An ultra-bright bowtie nanoaperture antenna probe was made via elegant nanofabrication technology directly at the apex of microscale tapered optical fibers, and the experimental test using individual fluorescence molecules as optical nanosensors has indicated an ∼sixfold enhancement on the single molecule fluorescence signal.²² In another work, a polymer tip was fabricated on the end of the fiber, and fluorescent enhancements were observed with the intensity increased about 7.66 times compared with the case of the flat cut fiber.²³ Physically, the fiber tip can increase the extension and intensity of the evanescent wave into the fluorescent layer deposited on the optical fiber, and tapering of the optical fiber has proved to be a useful method to improve the collection efficiency of the fluorescence signal.²⁴ A novel U-shaped fiber-optic evanescent-wave fluorescent immunosensor was designed that excited by transmitting light into an optical fiber with an angle offset allows a much higher number of total-internal-reflections, which strengthens light–matter interactions. The results show that the fluorescence sensitivity of the U-shaped fiber-optic probe with light-sheet skew rays’ excitation is 16 times higher than that of collimated skew rays’ excitation.²⁵ 

In this paper, we report the design and fabrication of an optical fiber bound microstructured spiral conical tapered gold tip exhibiting an excellent PEF effect as a promising fluorescence detection device. The overall geometric and physical configuration of the fiber-bound spiral-grating conical tapered gold tip, which is used to induce PEF of molecules, is illustrated in Fig. 1(a). The gold tip tightly bound to the end face of the single-mode optical fiber can be fabricated via the combinative manufacturing scheme of direct laser writing (DLW) 3D printing technology for core template building and magnetron sputtering technology for conformal gold film coating (with the details described in the supplementary material).¹⁷ As depicted in Fig. 1(b), the 3D gold tip has a spiral-grating conical tapering geometry, which can be looked upon as a gold grating decorated at the surface of a conical tapered tip. The surface of the tip is a one-dimensional periodic grating along every conical line; yet in the cross section, there is no mirror-reflection symmetry. The peculiar geometry of this spiral-grating conical tip allows for efficient coupling and transport of energy for any input light signal with arbitrary polarization states either linear, circular, elliptical, vectorial or other more complicated polarization configurations according to previous studies.¹⁷ Therefore, one does not need to take care of the incident light signal polarization state.

As depicted in Fig. 1(c), the input laser light comes from the far end of the optical fiber, transports to the gold tip, tunnels through the outer conical-grating gold film, excites the surface plasmon, induces great local electric field enhancement, and triggers bright fluorescence of molecules at and around the surface of the gold tip. To see at what geometric parameter of the gold tip one can achieve as much as possible local field enhancement due to efficient excitation of SPP (Surface Plasmon Polariton) at the outer surface, we have made detailed three-dimensional finite-difference time-domain (3D FDTD) numerical simulations upon the optical near-field distribution of the 3D gold tip at various tapered angles from which the optimized geometric parameters for the purpose of PEF can be extracted. E-field intensity distributions in the x–z plane for the gold-coated spiral-grating tip with different half-tapered angles at the incident light wavelength of 532 nm are shown in Fig. 2. Supposed that the half tapered angle is θ. In Figs. 2(a)–2(d), we have considered and compared four tapered angles as θ = 45°, 40°, 30°, and 20°.

A first glance at four optical near-field distribution patterns can tell us that the tip with θ = 30° has the best performance in terms of exciting SPR and enhancing the optical field at the surface of the gold tip. A detailed comparison upon the E-field distribution and calculation of the ratio of the average electric field strength of the outer cone surface to the incident light field (E/E₀) is about 98.5, 192.7, 1947.2, and 1532 V/m for the half angle of θ = 45°, 40°, 30°, and 20°. Here, the incident field is assumed to have an amplitude of 1.0 V/m. It can be obtained that the spiral tapered with the half angle of both 20° and 30° has the stronger field enhancement, while the E-field enhancement of 30° is stronger than that of 20°, 40°, and 45° by about 1.27, 10.1, and 19.8 times. The simulation results indicate that the spiral tapered angle (θ = 30°) with metal grating (period of 680 nm) can satisfy the surface plasmon excitation condition.

In order to achieve such a design through experiments, we adopt the DLW method based on two-photon polymerization technology^26,27 achieved via a commercial instrument (Nanoscribe Photonic Professional Option GT) to directly 3D print the gold tip on an optical fiber end face. The setup of fabrication is schematically shown in Fig. 3(a). First, we put a drop of photoresist on the glass substrate and fix the processed fiber end face in the photoresist. Due to the 3D point-by-point manufacturing feature of DLW, arbitrary 3D microstructures can be manufactured by scanning the focused laser points in the photoresist using a programmed route. After post-processing, to remove the residual photoresist free from two-photon polymerization, we obtain a solid spiral tapered conical tip made purely from the photoresist. Then, as illustrated in Fig. 3(b), we use this polymer tip as a template for conformal coating of a ∼80 nm thick smooth gold thin film by the classical magnetron sputtering technique using an optimized coating speed. After such a two-step fine-precision manufacturing procedure, the optical fiber-bound spiral conical tapered gold tip composed of a smooth and regular grating-like polymer-gold core–shell microstructure is created. The SEM pictures of the fabricated microstructured gold tip are displayed in Figs. 3(c)–3(f). From the enlarged SEM image as illustrated in Fig. 3(e), showing the detailed geometric configuration of the fabricated spiral tapered gold tip, the period of helix is about 680 nm and the width of the grating corrugation structure is 540 nm. Therefore, this two-step fabrication method is still simple and effective for building the current specific polymer–metal composite microstructure.

Figure 4(a) illustrates the schematic diagram of the fluorescence measurement setup for the spiral tapered gold tip bound optical fiber against RhB dye molecules. The fluorescence spectra are excited by using a 532 nm laser source, which is the best excitation wavelength of Rodamine dye molecules. The laser intensity has been adjusted to acquire an optimized fluorescence signal while minimizing the photo bleaching of dye molecules. The beam is coupled into the optical fiber through a 40× objective lens, and the output signals of the optical fiber are collected by a 20× objective lens. As a control experiment, we consider an optical fiber with its end face coated with a flat-cleaved 80 nm thick gold film and compare the fluorescence performance of this control sample with the spiral tapered gold tip bound fiber sample. Figure 4(b) illustrates fluorescence emission spectra from the spiral tapered gold tip sample and the flat-cleaved gold film sample. Note that two spectral curves have been carefully calibrated for the sake of precise comparison. First, the input laser intensity for two optical fiber samples maintains the same value. Second, the coupling efficiency of the input laser into two optical fibers is adjusted to be the same value. Finally, the number of dye molecules adsorbed uniformly on two samples is the same. A significantly large enhancement factor of molecular fluorescence is obtained on the spiral tapered gold tip sample compared with the flat-cleaved gold film control sample, which reaches up to ∼23-fold. We have taken the CCD images against the end face of two fibers, which are shown in Figs. 4(c) and 4(d). The flat-cleaved gold film sample has the orange-red fluorescence spot only concentrated at the center of the optical fiber with a diameter of about 20 µm in good agreement with the fiber mode size. In comparison, for the spiral tapered gold tip sample [Fig. 4(d)], a very bright orange-red fluorescence spot is observed to locate at the center of the optical fiber with a size almost the same as the gold tip bottom diameter. In addition, the entire outer flat end face of the optical fiber beyond the central gold tip, which is also coated with a 80 nm thick gold thin film during magnetron sputtering, also exhibits apparent orange-red fluorescence and the brightness is comparable with the central fluorescence spot of the flat-cleaved gold-film fiber sample. We attribute this feature to the efficient excitation of the surface plasmon polariton in the outer shell gold film by the 532 nm laser light laterally scattered from the central 3D microstructured gold tip, as a result of which dye molecules adsorbed in these outer shell regions are also excited to emit the fluorescence signal.

We have further tested the total number of photons in the 0–12.5 ns time window of two optical fibers at different incident intensities for comparison. The experimental results are shown in Fig. 5, which indicate that when the incident light is 0.5, 0.67, 1, 1.5, and 3 µW, the spiral tapered gold tip optical fiber has the dye molecule fluorescence intensity about 38, 34.5, 23, and 20 times, respectively, greater than that of the flat-cleaved gold film optical fiber. It is obvious that at all levels of illumination laser intensity, the spiral tapered gold tip sample exhibits a far stronger dye molecule fluorescence signal intensity than the flat-cleaved gold film sample. In addition, the enhancement factor gradually decreases when the illumination laser intensity increases. This might be due to the increased strength of non-radiative transition and photo-bleaching of dye molecule excited states caused by the increased photothermal effect of the gold film, which becomes much more pronounced for the microstructured gold tip due to the surface plasmon resonance than the flat gold film at higher incident laser power.

In conclusion, we have used the DLW system to fabricate a spiral-grating conical tapered gold tip bound to the end face of the optical fiber. We have made a deliberate design on the optimized geometry via 3D FDTD numerical simulations at which the gold tip allows the incident light coming from the optical fiber to efficiently excite the surface-plasmon polariton at the outer surface and trigger bright fluorescence of Rodamine dye molecules. Experimental results show that when the incident green laser light at 532 nm has an intensity of 0.5 µW, the fluorescence enhancement factor by the spiral tapered gold tip binding fiber is about 38 times greater than the reference sample of the flat-cleaved fiber with the roughened gold thin film deposited at the end face. This 3D nanostructured gold-tip bound optical fiber may provide a promising way to improve the detection sensitivity in fluorescence-based sensing platforms. As the manufacturing and analysis procedure is easily adaptable and inexpensive, this scheme of PEF fiber optical sensors allows for portable, user-friendly, and remote operation, may become a competitive candidate for compact fluorescence spectroscopy, and can find potential applications in biotechnology, clinical analysis, and analytical chemistry.

See the supplementary material for details regarding the sample fabrication of the optical-fiber-bound microstructured gold-tip, FDTD simulations, and fluorescence characterizations and measurements.

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 11974119), the Science and Technology Project of Guangdong (Grant No. 2020B010190001), the Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06C594), and the National Key R&D Program of China (Grant No. 2018YFA0306200).