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Abstract
We report on the micro-pattering of fluorescent gold nanoclusters embedded in polyvinyl alcohol film using a low-cost continuous-wave laser. We show that the formation of gold nanoclusters requires low-level laser irradiation (<100 W/cm2), corresponding to a sub-microwatt focused beam. This low-intensity requirement enables substantially higher speed and cost-effective light patterning process. As a result of using only small intensities during gold nanoclusters production, no microscopic topological changes were observed in the polymer film. This class of gold nanoclusters potentially finds application in high-speed authenticity marking and similar labeling applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Noble metal nanoclusters (NCs) —fluorescent nanodots with sizes smaller than 3 nm [1,2]—have been the subject of much interest over the last few decades [3–6]. Owing to their ultra-small size, NCs possess discontinuous band structures with quantized energy levels, leading to distinctive optical, electrical, and chemical properties compared to their larger nanoparticles counterparts [7,8]. These molecule-like features include strong absorption, intense fluorescence, long lifetime, large Stokes shift, broad spectral tunability, good photostability, biocompatibility, and facile synthesis, making this class of fluorophores a leading candidate for various applications, such as sensing [9], imaging [10] and biological labeling [11,12].
There exists a vast amount of synthesis pathways to generate and stabilize NCs [2,4–6]. One typical way to induce the formation of NCs is by using a bottom-up approach, which is reliant on the reduction of metal ions to neutral states via chemical reductants (e.g., Sodium borohydride) [13], thermal annealing [14], light (e.g., ultraviolet lamp or laser irradiation) [15,16], or $\gamma$-rays [17], leading to aggregation of metal atoms. However, unless the reduction occurs within proper stabilizing templates (either organic or inorganic materials), aggregation of atoms continues, resulting in irreversibly large non-fluorescent particles. Examples of stabilizing agents reported are thiols, dendrimers, polymers, DNA oligonucleotides, peptides, proteins, glass and zeolite [4,6,18].
Meanwhile, direct laser writing (DLW) has been used as a powerful and robust tool to permanently modify the refractive index of transparent materials and fabricate sub-diffraction-limit micro-structures, which have found applications in photonic devices [19–22]. DLW has been also employed to locally photo-reduce metal-ion-doped substrates to fabricate structures containing metallic nanoparticles [8,23,24]. Besides, there exists a considerable body of literature on the formation of brightly emissive silver nanoclusters (AgNCs) through DLW of silver-ion-doped templates such as phosphate glass [17], zeolite [25], and more recently polymer [26,27]. Such entities hosted in solid-state matrices display excellent chemical- and photo-stability, and are shown to be good enablers for applications as optical data storage [28], optically encoded microcarriers [25], and authenticity micro-labels [29]. In general, the above-mentioned usage of DLW is based on multi-photon absorption, which requires expensive femto- or pico-second lasers with large pulse energies [26,28]. We have previously shown that DLW of AgNCs using low-cost, continuous-wave (CW) laser beams within poly(methacrylic acid) (PMAA) [27,29], and polyvinyl alcohol (PVA) [30] templates is also plausible. However, the high-intensity laser light required to produce polymer-scaffolded AgNCs tends to damage the polymer films and leaves visible grooves on the written regions.
Despite the interest in the production of localized patterns of fluorescent metal NCs [26,28,29], previous works have tended to concentrate on AgNCs and investigation of other noble metals have been so far unseen opportunity. Among all types of metal NCs, gold is of highest importance since it is less toxic, more biocompatible, and has relatively bio-inert surfaces and extraordinary stability [7]. In addition, modifying gold surfaces is generally more straightforward and allows for more control over the clusters’ size and shape [2,5,18]. Consequently, gold nanoclusters (AuNCs) with broad emission spectrum provides a robust platform for many applications.
In this paper, we demonstrate the formation of localized fluorescent AuNCs in polymer films by DLW. We use a low-cost CW laser with wavelength of 473 nm to activate AuNCs in PVA films, and we explore the evolution of NCs during the laser irradiation. We also study the optical properties and fluorescence spectra of the AuNCs. We find that the formation of AuNCs requires merely a few tens of W/cm$^2$ laser intensity, which corresponds to sub-microwatt laser power when focused down to micrometer scale. Owing to the low-level irradiation the polymer film undergoes no damaging effect as opposed to silver-containing polymers. This property enables micro-patterning of fluorescent features with a high-speed, cost-effective, and facile technique, making it more suitable for the applications mentioned above.
2. Methods
2.1 Sample preparation
We dissolved 240 mg (5 mmol) of polyvinyl alcohol (PVA) powder (Sigma-Aldrich, M$_{\textrm {w}} = 89,000-98,000$, $99+\%$ hydrolyzed) in 8 ml of water to obtain 3 wt$\%$ PVA solution. Next, different quantities (0 to 640 mg $\approx$ 1.6 mmol) of gold(III) chloride trihydrate (Sigma-Aldrich, $> 99.9 \%$) were dissolved in 8 ml of water. The polymer and gold-precursor solutions were subsequently mixed, resulting in 1.5 wt$\%$ PVA solutions with different Au/OH ratios (i.e., number of gold atoms to number of hydroxides, from 0 % to 30 %). The blends were subsequently spun cast on round borosilicate coverslips (ø25 mm $\times$ 0.16 mm), with 1500 RPM and for 2 minutes. Finally, samples were stored in a nitrogen desiccators cabinet to dry, resulting in 50-nm-thick PVA films containing gold precursor.
2.2 Optical setup
Schematic illustration of the laser writing procedure for Au-PVA films and the fluorescence spectroscopy setup is shown in Fig. 1. The polymer-coated substrate was flipped over such that the polymer film was sandwiched in between the coverslip substrate and a glass microscope slide, and was fixed on a three-axis motorized scanning stage (Thorlabs, MAX303/M). CW laser beam from a single-mode laser diode (Cobolt 06-01 Series) with a wavelength of 473 nm was cleaned and expanded to obtain a collimated beam of 6 mm diameter using a spatial filter system. A polarizing beamsplitter cube, housed in a rotation mount, was utilized to adjust the laser output power. The resulting beam reflected off a dichroic mirror (Semrock, FF484-FDi01-25x36), was redirected via a 45-degree-mounted mirror and was delivered to the sample through an oil-immersion objective lens (Leica HCX PL APO 100X/1.4 NA). This lens was responsible for delivering the excitation light and for collecting the emission from the specimen. The laser beam was focused onto the back aperture of the microscope objective through a convex lens with a focal length of 200 mm, resulting in a collimated beam with ≈60 µm diameter on the film. In this way, we could produce and excite a larger population of NCs. For patterning purposes, the aforementioned convex lens was removed, and a collimated beam was delivered to the microscope objective, thus making a focused beam spot on the polymer film, with a diameter of about 200 nm. A shutter (Thorlabs SH05) along with a controller (Thorlabs SC10) was utilized in the beam path to control exposure times. Collected fluorescence from AuNCs was passed through the dichroic mirror and was redirected to a bandpass filter (Semrock, FF01-612/69-25) and was focused onto an EMCCD camera (Andor, iXon3 897) through an achromatic doublet lens with a focal length of 200 mm. The fluorescence was redirected to a spectrometer (Avantes, AvaSpec 2048) by a flipping mirror or a beamsplitter to measure the spectra. To investigate the effect of wavelength on the writing process, a supercontinuum white light source (Fianium Ltd.) was coupled to a computer-controlled monochromator (HORIBA, iHR550), with slit width set to 2 mm corresponding to the spectral resolution of 12 nm.
Fig. 1. Schematic illustration of laser patterning process on Au-PVA thin films. The desired ratio of gold chloride and PVA sources was dissolved in water, and the resulting aqueous solution was spun cast on a glass coverslip and dried to find a thickness of 50 nm. A CW laser beam with a wavelength of 473 nm was focused on the film to activate localized fluorescent patterns of AuNCs. The components are lenses (L1, L2, L3, L4, and L5), polarizing beamsplitter cube (PBS), beamsplitter cube (BS), mirror (M), dichroic mirror (DM), and microscope objective lens (O).
2.3 Scanning transmission electron microscopy
A thin Au-PVA film with 10 % Au/OH ratio was spun cast on TEM grids (Agar Scientific, AGS147-3H) with 2500 RPM for 2 minutes to guarantee film thickness of $<50$ nm, which is crucial for TEM measurements. A second step spin, set at 4000 RPM for 30 s, was immediately run to cast off the edge beads, thus preventing film quality deterioration. Several grid cells were selectively exposed to laser irradiation for sufficient time to reach maximum fluorescence intensity. The STEM photography was performed at the Nanomicroscopy Center in Aalto University, using JEM-2200FS double Cs-corrected electron microscope (JEOL Ltd) operated at 200kV.
3. Results and discussion
3.1 Evolution of nanoclusters
Understanding the effect of laser irradiation on Au(III)-containing PVA films and the evolution of the fluorescent AuNCs is a prerequisite to utilizing these nanocomposite films in laser patterning applications. To this end, a collimated laser beam with 473 nm wavelength was launched to strike the polymer film while the growth and decay of the emission were traced by in-situ imaging of the sample using an EMCCD camera. Figure 2(a) shows series of fluorescent images captured at different times, during which the polymer film was irradiated with a laser beam having a Gaussian profile and a peak intensity of 100 W/cm$^2$. As shown in Fig. 2(a), the fluorescence intensity evolves with radial gradient, with smaller rise time ($\tau _{\textrm {rise}} \approx 15$ s) and decay time ($\tau _{\textrm {decay}}\approx 50$ s) at the center, where the peak intensity of the laser beam resides. Given the Gaussian profile of the laser beam, the NCs evolution for different excitation intensities can also be calculated. This estimation was performed by extracting fluorescence intensities of 1.28 µm x 1.28 µm squares at different parts of the images. Figure 2(b) illustrates a selection of the resulting evolution curves consisting of two regimes, i.e., rapid growth and gradual decay. Comparing these growth plots with the evolution of AgNCs in polymer within similar time scales, we conclude that the required intensity to generate fluorescent AuNCs (which is tens of W/cm$^2$) is about three orders of magnitude smaller than that of AgNCs (which is tens of kW/cm$^2$) [27,30]. Furthermore, each graph in Fig. 2(b) exhibits a logistic growth in the emission intensity, which is correlated with the population of AuNCs in the corresponding region. Therefore, the emission intensity growth can be stated as $I = I_{\textrm {max}}/(1+e^{-k(t-t_0)})$, where $I_{\textrm {max}}$ is the maximum fluorescence intensity, $k$ is the steepness of the curve, and $t_0$ is the time to reach to the midpoint of the peak. The higher the laser writing intensity, the faster the rate at which AuNCs are generated. Likewise, the higher the excitation intensity, the faster the rate at which the fluorescence of the written AuNCs decays. This decay is likely due to the growth of the AuNCs into larger nanoparticles which are not fluorescent.
Fig. 2. (a) False-color fluorescence images of an Au-PVA film with Au/OH ratio of 20$\%$, captured at different stages while the sample was exposed to a laser irradiation with 473 nm wavelength and maximum intensity of <100 W/cm$^2$. (b) Fluorescence intensity evolution of 1.28 µm × 1.28 µm square regions of Au-PVA film exposed to different laser intensities. The inset represents the dependence of the NCs formation rate (blue circles) and the fluorescence intensity decay rate (red triangles) with respect to the laser intensity.
We define $\kappa _{\textrm {rise}}$ and $\kappa _{\textrm {decay}}$ as the rates for the generation of AuNCs and the decay rate of the fluorescence intensity, which are the inverse of the rise time and the decay time, respectively. The inset in Fig. 2(b) displays a plot of $\kappa _{\textrm {rise}}$ and $\kappa _{\textrm {decay}}$ versus the laser intensity, based on which $\kappa _{\textrm {rise}} \approx 3.5 \times \kappa _{\textrm {decay}}$ for a given laser intensity. Therefore, by stopping the irradiation at the peak of the fluorescence intensity and exciting the as-formed AuNCs with lower intensities, the decay rate becomes negligible. It should be noted that the higher peaks in Fig. 2(b) are due solely to the higher excitation intensity, and the population of AuNCs is supposed to be comparable in all curves once the evolution reached a summit. This can be corroborated by the linear relation of the peak emission intensity to the excitation intensity. Of note, whereas the required intensity for activating AuNCs in the polymer film is very low, it is at least two orders of magnitude higher than the intensity of solar irradiance, which is about 0.1 W/cm$^2$ [31]. Therefore, the Au-PVA film is not sensitive to the ambient light, and no AuNCs can be activated through exposure to sunlight. Similarly, the decay time exceeds a few minutes provided that the written features are excited by intensities less than 20 W/cm$^2$, which is again significantly higher compared to the intensity of solar irradiance.
The photochemical reactions involved in the DLW of Au-PVA films can be explained as follows. The laser irradiation cleaves some polymer bonds, which yields radical intermediates with strong redox capabilities [15,32,33], thus first reducing Au(III) to intermediate gold species and finally to Au(0). Besides, the reduction can also be induced through the PVA, which is a well-known mild reducing agent [34,35]. In this case, the secondary OH groups of the polymer chains are oxidized to ketones, and Au(III) ions are reduced to Au(0) [32]. This chemical-reduction mechanism is usually initiated through a baking process [34]. Following the reduction, H$^+$ is released as a by-product, inducing PVA cross-linking [33].
To explore the effect of the laser wavelength on the writing process, a supercontinuum laser beam was coupled into a computer-controlled monochromator whose output light, with a maximum intensity of about 30 W/cm$^2$, was impinging a sample with Au/OH ratio of 20 %. The evolution of the fluorescence intensity as a function of the excitation wavelength was traced by in-situ imaging of the sample. Figure 3 demonstrates normalized fluorescence intensity evolution exhibited by 1.28 µm x 1.28 µm square regions of the sample for violet, blue, and green excitation lights. While shorter wavelengths cause faster growth and decay in fluorescence intensity, longer wavelengths result in a significantly slower formation of NCs, and in turn, longer decay time. This could be due to the fact that shorter wavelengths are closer to the absorption band of AuCl$_4^{-}$, and are absorbed by the film more efficiently. Therefore, violet light would be the most efficient and fastest for DLW and activation of AuNCs, while green light enables efficient reading as AuNCs display superior photostability at this wavelength range.
Fig. 3. Normalized fluorescence intensity evolution pertaining to 1.28 µm x 1.28 µm square regions of an Au-PVA film with 20 % Au/OH ratio, exposed to light beams with different wavelengths. The light intensity was set to be around 30 W/cm2 for all wavelengths.
3.2 Optical characterization
We further investigated the evolution of the AuNCs emission spectra for Au-PVA films under laser irradiation. To this end, Au-PVA films with different Au/OH ratios (from 2.5 % to 30 %) were exposed to a collimated laser beam of 60 µm diameter, while emission spectra were recorded on 100 ms time steps. Figure 4(a) shows the emission spectra of an Au-PVA film with 20 % Au/OH ratio which was exposed to a beam of 700 W/cm$^2$. As can be seen, upon exposure, fluorescence intensity grows in time logistically (as described above), reaching a maximum after being irradiated for 3 seconds. Subsequently, as irradiation continues, the emission enters to a decay phase (see Fig. 4(b)). Interestingly, there is no visible change in the profile of the spectra from the beginning of the growth and after the decay, suggesting that the sources of fluorescence remain unchanged. The broad spectrum from 500 nm to 800 nm may be attributed to the fluorescence obtained from AuNCs. The dichroic mirror prevents lights with wavelengths $<500$ nm reaching the detector, thus accounting for the sharp cutoff in the emission spectra at 500 nm.
Fig. 4. Emission spectral (a) growth, and (b) decay of AuNCs embedded in PVA film with 20 % Au/OH ratio. The spectra were recorded at different stages when the film was irradiated with a laser beam of 60 µm diameter and maximum intensity of 700 W/cm$^2$.
Given the broad and asymmetric experimental line shapes, one could predict that the emission spectra of PVA-encapsulated AuNCs consist of more than one component. Accordingly, to deconvolute photoluminescence curves, we employed a two-Gaussian fitting model. Figure 5 illustrates the experimental emission spectrum (blue symbols) corresponding to a film with 30 % Au/OH ratio, where two constituent Gaussian components are shown (dashed and dash-dotted lines), with peaks at 560 nm and 660 nm, and FWHMs of 100 nm and 115 nm, respectively. The entire emission spectra are well replicated (solid red line) by summing up the two constituent components, with a coefficient of determination being $R^2 \approx 0.998$. The consistent lineshape of the spectra during the evolution and decay suggests that the fluorescence from AuNCs is entirely size-independent, in which visible emission could be mainly from Au(I) in the clusters, as reported in Refs. [5,16]. Inset in Fig. 5 is a plot of the peak intensity emission acquired from AuNCs corresponding to samples with different Au/OH ratios. As expected, higher concentration of gold precursor resulted in brighter features of AuNCs.
Fig. 5. Emission spectrum (blue symbols) of AuNCs within an Au-PVA film with 30 % Au/OH concentration, excited by 473 nm laser with 60 µm spot diameter and maximum intensity of 1.7 kW/cm$^2$. The emission profile was fit using a two-Gaussian model (red line), where the dashed and dash-dotted lines are the constituent elements. Inset depicts the relation between fluorescence intensity of AuNCs against Au/OH ratio.
3.3 Scanning transmission electron microscopy
Advances in transmission electron microscopy (TEM) techniques enables to directly probe the geometry of fluorescent NCs with resolutions down to the atomic levels [36]. To prepare samples for TEM, thin films need to be deposited on fragile grids and be exposed to the laser beam. We prepared the grids by coating them with thin polymer films and exploited scanning transmission electron microscopy (STEM) to obtain detailed nano-scale information about the changes in Au-PVA film sites after being exposed to the laser irradiation. Figure 6(a) shows a photograph corresponding to a PVA film containing Au(III) with no exposure to laser light. As can be seen, numerous sub-nanometer gold dots already reside in the film (see Fig. 6(b)). These dots could be formed during the sample preparation, as a result of the reduction ability of PVA, and were kinetically trapped in the film. In contrast, Fig. 6(c) represents a STEM image related to sites of the Au-PVA film which were subject to laser irradiation. The laser exposure was performed by in-situ imaging of its fluorescence and was stopped once the emission maximized. These regions contained NCs with sizes mostly between 2 nm to 3 nm. The histogram in Fig. 6(d) shows the particle size distribution estimated from several images of regions experienced similar exposure to laser light. Energy-dispersive X-ray spectroscopy was also used to confirm the gold composition of particles (data not shown). Owing to the size distribution, estimated from STEM images, together with the size-independent visible emission spectra (discussed above) we hypothesize that the luminescence in Au-PVA films is originated from the fraction of Au(I) ions in clusters [5,16].
Fig. 6. (a) A STEM image of a gold-precursor-loaded PVA film with 10 % Au/OH ratio before exposure to laser light. (b) Relative size distribution of AuNCs corresponding to the image shown in (a). (c) A STEM image of an identical film as in (a) but after the exposure to the laser light. (d) Relative size distribution of AuNCs corresponding to several STEM images of exposed Au-PVA film with identical Au/PVA concentration, exposure time and intensity as in (c).
3.4 Micro-patterning
We made fluorescent micro-patterns of AuNCs by focusing the laser beam into Au-PVA films using a 100X oil-immersion microscope objective with NA of 1.4. The sample was moved using a 3D translation stage. The laser power was reduced to micro-watt levels to provide suitable intensity for DLW, preventing immediate decay in the fluorescence intensity of the as-formed AuNCs. The exposure time for certain intensities was also controlled by adjusting the speed with which the sample was scanned. Emission images of laser written patterns are depicted in Fig. 7, where the DLWs were performed using a 473 nm laser. Figure 7(a) corresponds to a film with 10 % Au/OH ratio, made by 2 µW power and 5 µm/s scanning speed, while Fig. 7(b) is associated with another film with 20 % Au/OH ratio, written by 0.5 µW power and 2 µm/s scanning speed. The images are results of excitation by a 60 µm-diameter collimated beam of the same laser used for DLW and are represented in false color. The brightest spots in the images (marked by white dashed circles) are related to the points where the film was adjusted to the focal spot of the laser beam, resulting in prolonged exposure and generation of more AuNCs in those areas. The linewidth of the patterns in these images is ≈1 µm, which is about five times larger than the beam spot. The linewidth could be improved by finely controlling the laser power, or by writing with higher speed. Studies on the relation of scanning speed and intensity to the brightness and linewidth of the fluorescent patterns are left for future work.
Fig. 7. False-color emission images of patterns generated by scanning Au-PVA films against a focused laser beam with 473 nm wavelength. Patterns were written within (a) a film of 10 % Au/OH ratio, with a laser power of 2 µW and a scanning speed of 5 µm/s, and (b) a film of 20 % Au/OH ratio, with a laser power of 0.5 µW and a scanning speed of 2 µm/s. A collimated beam of the same laser used for DLW was exploited to excite the patterns. The white dashed circles indicate the points where the film was aligned with the focal spot of the laser beam.
The fluorescent patterns in Fig. 7 display intensities 4 times above the background image. The brightness and contrast can be further improved by preparing thicker PVA films. Apart from film thickness, brightness is also dependent on the concentration of gold source used in the polymer film (as shown above). Therefore, by increasing the concentration of gold, the brightness is improved. It is worth mentioning that the required light intensity to activate NCs in Au-PVA films is far below the threshold at which polymer film undergoes microscopic topological changes. Hence, contrary to that reported for AgNCs [26,30], no grooves were observed through bright field microscopy after DLW (results not shown). This property is particularly advantageous for applications in anti-counterfeiting where the secret information –in the form of bar codes or quick response (QR) codes– is fluorescent under a special light source while invisible under ambient lights [37]. In addition, due to resolution being limited by the diffraction of light, DLW of Au-PVA films enables significantly smaller fluorescent features as compared to those made through inkjet printing technique [38]. Finally, thanks to the biocompatibility of gold and PVA, DLW of Au-PVA films could be used to fabricate fluorescent micro-taggants for in-dose drug and food authentication, and other anti-counterfeiting applications [39].
4. Conclusions
We have demonstrated that the DLW of a PVA film loaded with gold precursor can produce fluorescent microstructures of AuNCs, with a broad emission spectrum, spanning over the visible wavelength range. Study on the evolution of AuNCs through fluorescence microscopy and spectroscopy confirmed that the required intensity level to activate AuNCs lies within a few tens of watts per square centimeter. While this intensity level is far higher than the solar irradiance, it is remarkably lower than the required intensity level for DLW of AgNCs in a similar film. Thus, this material system, together with the facile and cost-effective method, could pave the way for a new type of fluorescent markers for authentication and anti-counterfeiting applications. Besides, we anticipate that this technique can also be exploited for the fabrication of AuNCs in different types of polymers containing gold salt.
Funding
Academy of Finland (310801; 320165).
Acknowledgment
This work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises. The authors thank Dr. Hua Jiang for STEM measurements.
Disclosures
The authors declare no conflicts of interest.
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