Surface Nanodroplet‐Confined Engineering of Gold (I) ‐Thiolate Nanostructures

Gold(I)‐thiolate complexes have served as primary building blocks for diverse Au nanostructure synthesis strategies. A delicate approach to characterize and control Au(I)‐thiolate motif formation and assembly on the surface is needed as it can potentially solve challenges associated with utilizing gold nanomaterials in many applications. Here, the controllable generation of flower‐shaped surface gold nanostructures (FSGNs) is demonstrated by manipulating the formation‐assembly process of Au(I)‐dodecanethiolate motifs within nanoscale surface droplets. The morphology and structure of the resulting Au nanostructures are governed by internal convection flows and interfacial energy, modulated by the nanodroplet composition and substrate wettability. The obtained FSGNs are proven to act as versatile scaffolds for the selective generation of Au spiky nanostars. These FSGNs can also be utilized to functionalize nanodroplet‐based reactors, boosting the fluorescent intensity of Nile red (NR) fluorophores and decomposing NR via catalytic reaction. Remarkably, with FSGN functionalized droplets smaller than a radius of 500 nm, the decomposition rate of NR can reach ≈0.01 s−1. These results highlight a miniaturized, controllable, and automated method for the in situ production of 3D gold nanostructures on substrates, offering prospects for fast surface nanostructure fabrication and efficient environmental pollutant treatment.


Introduction
As paramount metal nanomaterials, gold-based nanomaterials feature advantageous physicochemical properties, [1][2][3] such as high chemical stability, pronounced plasmonic effect, tunable optical properties, good biocompatibility, and high sensing specificity. [4,5]Hence, Au nanomaterials have been widely utilized in electronics, catalysis, solar cells, thermal ablative therapy, sensing, imaging, and many other applications. [6]mong various synthesis strategies, the specific reactions and interactions between gold and thiols, i.e, forming Au(I)-thiolate motifs (-S-Au(I)-S-), have been considered as building blocks to construct diverse Au nanostructures with distinct functionalities.For example, Au(I)-thiolate complexes using glutathione have been regarded as key intermediates for Au nanoparticle formation, displaying surface-enhanced Raman scattering (SERS). [7]The reactions of Au(I)-thiolate motifs with strong reducing agents can generate luminescent thiolate-protected gold nanoclusters, containing one to several gold atoms with exceptional catalytic performance. [8]In both scenarios, the internal structures of as-formed Au(I)-thiolate complexes have been considered as determining points for generating uniformly-sized Au nanoparticles and nanoclusters. [8,9]ntrinsically, Au(I)-thiolate complexes are classified as coordination polymers.[12][13][14][15] Mesoporous gold nanospheres can also be gained via self-assembly of Au(I)-thiolate complexes. [10]The current protocols to acquire Au nanostructures have been largely based on controlling Au(I)-thiolate formation in bulk solutions.A typical reaction involves forming Au(I)-alkanethiolates (Au(I)-SR) and disulfide (RSSR) from Au(III) ion and alkanethiol (RSH) following the reaction of Au (III) + 3RSH → [Au (I) − SR] + RSSR. [16,17][19] Among them, mesomorphic Au(I)-octadecanethiolate assemblies were regarded as having the most ordered bilayer assembly structures and strongest luminescence ascribed to significant aurophilic interactions of Au(I)-Au(I).Similar lamellar structures were also found in Au(I)-glutathione species to facilitate the generation of supramolecular hydrogels. [20]][29] Moreover, polydispersity and structural alteration often arise when these solution-based Au nanomaterials are transferred to be immobilized onto substrates for nanofabrication, catalysis and sensing. [30]n-situ forming Au nanostructures on surfaces provides alternative strategies to alleviate the abovementioned issues.In addition to chemical vapor deposition, [31] photolithography, [32] electrochemical deposition methods, [33] spray or droplet fusion method, [34] the lately established surface-nanodroplet method offers high flexibility and low cost for nanomaterial synthesis and deposition. [35,36]Nano/micro-sized surface droplets are regarded as stationary droplets sitting on substrates, which are immersed in an immiscible liquid phase.[39] Accelerated interfacial reaction rates for hydrogen nanobubble production can be achieved due to the high surface area-to-volume ratio of surface nanodroplets, high local concentrations of reagents and more efficient thermal exchange. [40][43] Although surface droplets have displayed their versatility in many aspects, the utilization of nanoscaled surface droplets for controllable gold nanostructure production remains unexplored.The understanding of the Au(I)-thiolate complex formation in such a confined liquid-solid system is the key but yet missing.
In this work, the in situ formation and assembly of Au(I)thiolate in surface nanodroplets were investigated for the first time.The reaction of HAuCl 4 with dodecanethiol-composed surface nanodroplets was adopted as a model system and the confined formation of Au(I)-dodecanethiolate (Au(I)-DT) complexes was monitored by total internal reflection fluorescence microscopy (TIRF).It is evident that the formation and assembly of Au(I)-DT motifs confined in surface nanodroplets is dominated by Au-S interfacial reactions as well as the internal mixing of fluids inside nanodroplets, which leads to the creation of flowershaped surface gold nanostructures (FSGNs).The morphologies and sizes of resulting Au nanostructures are also determined by the nanodroplet composition and the substrate wettability.The cost-effective, robust, and controllable approach presented herein could open new avenues to advance many applications.As proof-of-concept example applications, we demonstrate that 1) the asformed FSGNs can be applied as scaffolds for fabricating gold nanostars and 2) the produced FSGNs can enhance fluorescent signals as well as in situ facilitate fast and efficient catalytic decomposition of Nile red in nanodroplets.

Formation of FSGNs by the Surface Nanodroplet Method
A stepwise synthetic process for surface Au nanostructures in a microfluidic channel is presented as a schematic diagram in Figure 1a.Briefly, surface nanodroplets involving dualcomponent, dodecanethiol-octanol are first formed on the substrate in the microfluidic channel by the solvent exchange process.The subsequent introduction of the HAuCl 4 precursors into the system leads to in situ interfacial reactions between dodecanethiol in surface droplets and AuCl 4 − ions from the surrounding aqueous solution.As one of the reactants, dodecanethiol comprises more than 90% of the droplet composition.The abundance of dodecanethiolmolecules relative to AuCl 4 −1 initiates the formation of [Au(I) − SC 12 H 25 ]. [17,18]Once the reaction was completed, the formed Au structures were taken out of the fluid cell and characterized under ambient conditions.
The optical images of droplets and formed Au surface nanostructures were captured and compared.TIRF images in Figure 1b represented the size and density of binary surface droplets obtained in the initial step under a solvent exchange flow rate of 400 μL min −1 .The probability density function (PDF) versus the radius of the droplets (R) was plotted accordingly.The mean radius of droplets was measured to be ≈610 nm. Figure 1c,d shows the size and morphology of the resulting nanostructures from the nanodroplets.Under the optical microscope, the nanostructures were observed to remain individually dispersed on the substrate, indicating their stability and preserved structures during the transit from the liquid phase to the air phase.The green-yellowish birefringent luminescence under polarized light in dark-field illumination (Figure 1c, bottom image) demonstrated the existence of uniaxial crystal structures.The scanning electron microscopy (SEM) images (Figure 1d, top) disclosed a flower shape of the as-prepared surface nanostructure, with wrinkled and hierarchical morphology.The base radius (R b ) of these nanostructures in SEM images was analyzed and plotted in Figure 1d, bottom.As shown, the R b ranged from several hundred nm to 1000 nm, consistent with the radius distribution of formatted binary surface nanodroplets (see Figure 1b).The average R b was found to be ≈487 nm, which is slightly smaller than that of the binary surface nanodroplets.The 20% deviation between droplet size and as-obtained nanostructure dimension is probably ascribed to the density change between liquid droplets and solid nanostructures as well as the retraction of the droplet contact line during the reaction.The other factor to be considered is that the different resolution limitations between TIRF and SEM techniques result in static data variation as the former technique cannot resolve objectives under 200 nm.Nevertheless, the results demonstrate that the surfacenanodroplets efficiently compartmentalized and acted as templates for the formation of FS-GNs.Of note, the size of the binary surface droplets can be controlled by adjusting the flow rates. [44,45]Figure S1 (Supporting Information) demonstrates that smaller droplets can be formed at a reduced flow rate.The average radius of produced surface droplets decreased to 480 nm when the flow rate (Q) was reduced to 100 μL min −1 .Therefore, it is anticipated that further miniaturized FSGNs can be fabricated accordingly.As quantified in previous studies [45] , the droplet size R scales with flow rate Q under the laminar flow conditions as R ∼ Q 1∕4 .In this system, the Q was tuned in the range of 100 -400 μL min −1 , leading to R in the range of 480 -610 nm.Accordingly, the dimension of the fabricated gold surface nanostructure, R b can be estimated to be in the range 384 -488 nm (considering 20% deviation).Additionally, the overall production rate can potentially reach 10 6 Au nanostructures per cm 2 within 30 min. [42]The presented approach herein is facile, controllable, and fast, which has great potential for vast micro/nanostructure fabrication and encoding applications. [42,43,46,47]urther surface element analysis by X-ray photoelectron spectroscopy (XPS) has revealed the presence of Au, O, S, and C on the FSGNs.The characteristic Au 4f 7/2 and Au 4f 5/2 orbitals in the high-resolution XPS are presented in Figure S2 (Supporting Information).Deconvolution of the Au 4f spectrum confirms the presence of both Au(I) and Au(0) components in FSGNs, with binding energies at 84.5 and 83.9 eV, respectively.The ratio of Au(I)/Au(0) was found to be 9:1, indicating the high content of Au(I)-thiolate on the FSGN surface.With full width at half maximum (FWHM) of ≈1.25 eV, the Au 4f 7/2 peaks at 84.5 eV, 0.6 eV higher than that of the Au(0) film, and 1.5 eV lower than that of sodium Au(I) thiomalate reference samples. [7,48,49]The shifting and broadening of the peak could result from the co-existence of Au nanoclusters and nanoparticles in FSGNs. [28]More accurate Au state characterization in the FSGNs can be complex and worthy of further investigation.

In Situ Observation of FSGN Formation Process Confined in the Surface Nanodroplet
To understand how the FSGNs were generated, the in situ formation of such surface Au nanostructures was monitored by TIRF microscopy.Representative time-dependent snapshots in the op-tical channel are displayed in Video S1 (Supporting Information).The appearance of black spots at the droplet fringes along with time elapsing represents the initiation of FSGN formation.Despite variation in the droplet size, a similar FSGN morphology was spotted at both large and small-sized droplets.Therefore, the formation dynamics within a surface droplet with R of ≈2000 nm were used as the representative case.Such a droplet was highlighted by the blue arrow in Figure 2a.In the first 400 s, a few spots (red arrows) protruded at the rim of the droplet and became more visible at 500 s, indicating the nucleation and growth of Au(I)-DT nanoaggregates [30] at the liquid-liquid interface.These spots then gradually grew to ≈300 nm and spread out toward the center of the droplet as the time increased to 600 s.This tendency is analogous to the observations in hydrogen nanobubble generation via reactive surface nanodroplets. [40]It is anticipated that the droplet rim with a low energy barrier, can provide favorable sites for the nucleation and growth of Au(I)-DT complexes.
Between 610 and 620 s, the formed nanoaggregates (highlighted by green arrows) were pushed from the middle to the periphery of the droplet.The movements herein revealed that convection flows were generated inside the droplet simultaneously when the interfacial dodecanethiol molecules of the droplets were consumed by AuCl 4 -.It is anticipated that the concentration of dodecanethiol molecules at the interface differed from their concentration at the interior of the droplet.This concentration gradient acted as a driving force to induce an upward flow inside the droplet.Moreover, as aforementioned, the reaction rate at the rim is faster than at other sites.The different reaction fluxes could promote outward capillary flow, which carried the nanomaterials to accumulate at the contact line of the droplets. [50]Circulatory solutal Marangoni flow can also happen as the octanol composition in the droplet is diffusive to the surrounding aqueous solutions, creating a surface tension gradient. [51]Another consideration is the solid surfaces, on which the nanodroplets nucleated and grew.Unavoidable surface defects can cause strong pinning at one side of the droplet edge, [47] inducing asymmetric flows, such as the rotation shown in the later stages of the Au(I)-DT assembly process.These complex flows synergistically promote the alignment and arrangement of the formed Au(I)-DT motifs during the reaction, as illustrated in Figure 2b, resulting in the 3D flower shape shown in Figure 1d.

Interfacial Energy Modulated Formation of FSGNs
To explore how to achieve controllable fabrication of FSGNs, the impacts of surface droplet composition and substrate wettability were investigated further.For the surface droplet composition aspect, when the droplet was formulated without octanol (Figure 3a,e), the resulting morphologies were polydisperse.Besides a few FSGNs formed, most of the surface was densely covered by irregularly shaped nanoaggregates with a mean size of around several nanometers.With the inclusion of octanol in the droplets, the resulting Au(I)-DT assemblies (Figure 3b,c,f,g) presented scattered uniform shape.This transition indicates that the existence of the amphiphilic molecule, octanol, can reduce the surface energy of surface nanodroplets [52] and provide better confinement for the Au(I)-DT complexes inside droplets.Interestingly, when the octanol was increased from 1 to 3 vol% in the droplet solution, instead of the flower shape, patchy structures composed of nanoclusters appeared on the substrate (Figure 3c,g).The patches followed the footprints of surface nanodroplets with an average size of 375 ± 140 nm.The nanoclusters inside are about 14 nm.
It has been reported that the morphology and size of Au(I)thiolates are dependent on the kinetics of the reactions.50 -200 nm hydrodynamic radius of Au(I)-thiolates were obtained when the reaction was carried out at 0 °C.In contrast, polydis-perse structures with size either less than 1 nm, within 50 -200 nm, or larger than 500 nm were observed when the reaction occurred at room temperature. [30]Slow reduction kinetics can also enable the formation of high-quality Au nanoclusters from Au(I)-thiolate complexes. [53]Our results clearly show the transition of the size distribution and structures, indicating the critical role of droplet composition in controlling the kinetic process of Au(I)-thiolate formation and assembly.
Different ratios of dodecanethiol and octanol in the solution can formulate different compositions of surface nanodroplets during the solvent exchange process.Our previous studies have built up a model to estimate the droplet composition from the oversaturation levels of the oil component in ethanol-water systems. [44]In general, the oil of higher oversaturation level, i.e., lower solubility, will be the major component in the multicomponent droplets.Here, the non-water-soluble component, dodecanethiol, dominates in the droplets compared to octanol.The relationship of dodecanethiol-octanol binary droplet composition with their original ratio in the liquid before forming droplets is listed in Table S1 (Supporting Information).The impact of droplet composition on the assembly structure formation is twofold.Firstly, the amount of dodecanethiol in the droplet determines the reaction rate.The higher the concentration of dodecanethiol in the droplet, the faster the production rate of Au(I)-DT complexes.Secondly, the existence of octanol in the droplet will lower the reaction rate, reduce the surface energy and facilitate internal mixing for different assembly structures.The synergetic effects from dodecanethiol and octanol ingredients in the nanodroplets lead to the different morphologies of Au nanostructures formed on the substrate.
As to the effect from the substrate wettability, with a fixed droplet composition, the morphology of Au nanostructures on a octadecyltrichlorsiliane (OTS) coated hydrophobic glass substrate (Figure 3d,h) was smooth cap shape with an average radius of 351 ± 203 nm, as compared to the rosette shape obtained on a hydrophilic glass substrate (Figure 3b,f).This difference implies that interfacial energy arising from the surface wettability can potentially alter the internal assembly structures of Au(I)-DT.One possible reason is that binary droplets on the hydrophobic OTS-glass substrate (with lower interfacial energy) have smaller contact angles than those on the hydrophilic glass substrate, as shown in Table S2 (Supporting Information).The decrease in droplet height will reduce the specific surface area of the droplet, thus decreasing the reaction rate, and suppressing the internal convection flows inside the nanodroplets during the reaction.The other consideration is that the hydrophobic-hydrophobic interaction between dodecanethiol alkyl chains with the OTS-Si surface promotes bilayer structures, templating the alignment of -S-Au(I)-S-into lamellar structures. [20]herefore, we revealed a controllable strategy to rationally create FSGNs by modulating interfacial energy in the confined system through adjusting nanodroplet composition and substrate wettability.The resulting hierarchical FSGN-functionalized surface could be advantageous to many technologies, including additive nanofabrication of sophisticated gold nanostructures as demonstrated below.
[56] The recently developed two-step surface symmetry-breaking approach can efficiently create gold superlattices with one side possessing nanocube morphology and the opposite side being nanostars. [57]n this approach, the first step is to construct partially thiolatedpolystyrene (PS)-capped Au surface nanostructures and to expose certain Au(0) sites for gold spike growth in the second step.To this end, self-assembly of PS-functionalized Au by drying sessile droplets followed by a careful UV-ozone treatment was required.
Inspired by this approach, we proposed a facile and simple way to use FSGNs as scaffolds for the selective growth of Au spiky nanostars.As shown in Figure 4, spiky structures are selectively grown on specific regions of FSGNs after drops contain-ing Au precursors, AgCl activation regents, and reducing agent of ascorbic acid (AA) were cast sequentially and mixed well onto the FSGN surface.The preferential growth of Au spiky nanostars indicates the existence of Au(0) in the FSGNs, consistent with XPS results.It is known that utilizing AgCl to active exposed Au(0) sites exclusively can facilitate the anisotropic growth of Au spikes. [57]The dark regions (pointed out by circles) without spiky nanostars are likely the Au(I) sites.As discussed earlier, created Au nanoassemblies can transit from Au(I)-thiolates to Au nanoclusters [58] by adjusting the composition of surface nanodroplets.With adequate amounts of dodecanethiol and octanol in the droplets, both Au(0) and Au(I) are likely to exist on the formed gold surface nanostructures, [28,59] hence providing partially exposed areas to form the spiky structure selectively.The example herein implicates the potential to fabricate asymmetric gold nanostructures by as-produced surface Au(I)-thiolate nanostructures.

Fluorescent Enhancement and Heterogeneous Catalysis in the FSGN-Decorated Surface Nanodroplets
In addition to decorating surface nanostructures, the in situ generated FSGNs can empower surface nanodroplets as a versatile platform by integrating multi-functionalities from gold nanomaterials and the femtoliter liquid of octanol.In the following, FSGN-functionalized surface nanodroplets for dynamical dye fluorescence (FL) enhancement and dye decomposition acceleration were demonstrated.A hydrophobic dye, Nile red (NR), was used as a model system because its participation coefficient of octanol/water is close to 1.
NR was introduced to the binary surface nanodroplets simultaneously by a solvent exchange step. [60]Owing to the fluorescence of NR, the initially formed nanodroplets containing NR can be detected by TIRF microscopy at 561 nm (excitation wavelength) channel.Once AuCl 4 −1 precursors were pumped into the microfluidic channel, the fluorescence intensity of each droplet changed along with the formation of Au(I)-DT complexes.The beginning condition was marked as time zero with an intensity of I 0 .The representative snapshots at different timing points are depicted in Figure 5a and the corresponding snapshots in the optical channel are shown in Figure S3 (Supporting Information).
The fluorescence of the droplets was initially observed to increase with time."Hot spots" with enhanced intensity located in the droplets were observed (indicated by a red arrow) at 360 s.The corresponding bright field images in Figure S3 (Supporting Information) show black dots appearing at the same position, indicating the nucleation and growth of the Au(I)-DT nanoasemblies.This FL intensity increase could be attributed to the fluorescence enhancement by surface plasmon resonance (SPR) of gold nanomaterials. [61]SPR describes an induced oscillation of electrons around the interface of metal-dielectric upon illumination at the wavelength of the plasma resonance peak, which can potentially increase the emission and or excitation rate of the dye in close proximity, hence promoting the enhanced dye fluorescence intensity.The key parameters to this SRP-induced fluo-rescence enhancement include nanomaterial geometry, size, and distance to the dye. [62]As discussed earlier, Au(I)-DT nanoaggregates were preferably generated at the rim of the droplets while dyes were confined inside the droplets with a radius of ≈450 nm and height of ≈120 nm.This configuration provided optimum space to distance dye molecules with Au nanomaterials.Therefore, the dyes near the base of the droplets were further lit as shown at 200 s in Figure 5a.With the expansion of Au nanoaggregates toward the top of the droplets, stronger localized enhancement, "hot spots", were created because of geometry and size factors.Au(I)-DT protruding from the top of the droplets can act as antennas, strengthening the SPR effect. [63]Meanwhile, larger-sized Au(I)-DT increased scattering, beneficial to the radiative emission of the dye. [64]However, the fluorescence enhancement ceased when Au(I)-DT assemblies further evolved.After 360 s, the intensity of the droplets started to fall.At 1000 s, there was barely any fluorescence to be observed.[67] The excitation and emission wavelengths of as-produced NR products are likely blue-shifted, which is beyond the detection range under in situ observation conditions.Hence, the fluorescence decrease was revealed.
The time-dependent relative fluorescent intensities (I/I 0 ) of surface nanodroplets were analyzed and plotted in Figure 5b.A "peak shape" profile was revealed for each droplet peaking at 360 s, indicating the same local concentration gradient at each droplet under the flow condition. [47]It is also possible that the size effect on the total duration of I/I 0 increase cannot be differentiated under the 10 s per frame imaging condition.Notably, the increase and decrease of the fluorescence showed the dependence on the droplet size as well.The smaller the size of the droplets, the sharper the rising or falling of the fluorescence intensity.Quantitively, the maximum intensity enhancement can reach more than 3-fold the initial intensity for the droplets with R ≤ 1000 nm and two-fold for the droplets with R > 1000 nm.As for the fluorescence decay process caused by the catalytic reaction, the following pseudo-first-order kinetic equation was used to estimate the kinetic rate constants. In where [C] is the concentration of NR in the droplets at time t, [C 0 ] is the concentration of NR in the droplets at time zero and K represents the kinetic reaction rate constant with the unit of s −1 .
The concentration of NR in each droplet was calculated to be 0.785 mM based on its bulk concentration and partition coefficient. [68]Under this low concentration, the fluorescent intensity (I) of NR is linearly proportional to its concentration ([C]).Therefore, When substituting Equation ( 2) into (1), we can obtain Equation (3) as By fitting the fluorescent decay curves with Equation (3), the kinetic reaction rate constant can be yielded with the values in the order of 10 −2 to 10 −3 s −1 .The regression correlation coefficient, R 2 , was larger than 0.995 for all the fittings shown in Figure S4 (Supporting Information).The correlation between K and the droplet size R is presented in Figure 5c.Significantly, the catalysis rate can be as high as 0.01 s −1 in the FSGN-functionalised surface droplets with R less than 1000 nm, which is 10 times faster than that of gold nanocluster-mediated catalysis [69] and a hundred or even thousand times faster than that of gold nanoparticle-based catalysis. [66,48]A more than ≈40% acceleration of K was observed in the nanodroplets with R ≤1000 nm, compared with that in the droplets with R > 1000 nm.The displayed droplet size-dependent performance implies that the confinement effect takes place in the surface nanodroplets. [70]Additionally, large specific surface areas and highly active sites could be imposed by the presence of hierarchical FSGN structures in these droplets, further promoting catalytic activities.The possible catalytic reaction is illustrated in the Figure 5c insert.The FSGN-functionalized surface nanodroplets could potentially benefit fluorescence-based diagnostics [61] and enable high efficiency of degradation catalysis. [69]

Conclusion
In summary, in situ synthesis of Au nanostructures with 3D spatial morphology was achieved in binary surface nanodroplets.The formation dynamics were first resolved by the TIRF technique, evidencing the existence of internal flows to induce the self-assembly process inside the surface nanodroplets.The interfacial reaction between AuCl 4 −1 and dodecanethiol, diffusion of octanol from the droplet to the surrounding environments, and the substrate characteristics were found to impact the flow motions inside the nanodroplets, determining how the Au(I)-DT complexes were packed, and hence influencing the morphologies of the final nanostructures.The composition of dodecanethiol in the surface nanodroplets mediated the reaction with AuCl 4 −1 , affecting the size distribution of Au(I)-DT complexes.Therefore, tunable Au(I)-DT nanoassemblies can be obtained by simply adjusting the droplet composition and substrate wettability.The achieved FSGNs were proven to be active growth sites for selectively producing Au spiky nanostars on top.In addition, the in situ formed FSGNs confined in surface nanodroplets have exhibited fluorescence-enhancement and catalytic abilities, due to the synergistic effect of the droplet compartmentalized process, and the shape of Au nanostructures.The findings provide new insights into confined nanomaterial synthesis and self-assembly in micro and nanoscale droplets, contributing to the in situ fabrication of advanced surface functional nanomaterials.The technique presented here can potentially benefit many nanotechnologies that require the immobilization of nanomaterials onto the surface, such as electrochemical catalysis, optoelectronics, and sensing device fabrications.Furthermore, the straightforward approach to functionalize droplets with nanomaterials endows a miniaturized platform with boosted efficiency for catalytic reactions, advancing techniques for environmental remediation and green energy utilization.
Formation of Surface Nanodroplets: The microfluidic channel with 0.4 mm height (sticky-Slide I Luer, ibidi company) attached to a precision glass substrate (26 × 60 mm, No. 1.5H, Marienfeld) was used to produce surface droplets.Prior to the attachment, the glass substrates were sonicated in ethanol for 5 min, dried by N 2 flow, and placed in a UV/Ozone cleaner (Bioforce NanoSciences, Inc.) for 15 min to maintain hydrophilicity.To produce surface binary nanodroplets, the flow channel was first filled with solution A containing 1 vol% dodecanethiol and octanol ranging from 0, 1, and 3 vol% in ethanol, and subsequently replaced by water at different flow rates from 100 to 400 μL min −1 controlled by a syringe pump (NE-1000, PumpSystems Inc.).In the case of droplets formed on hydrophobic glass substrates, OTS was coated onto the surface of a glass substrate by chemical vapor deposition as described in the previous work. [41]abrication of Surface Au Nanostructures: After the surface nanodroplets were produced on the substrate in the microfluidic channel, HAuCl 4 •3H 2 O aqueous solution (0.8 mM) was pumped into the system at the flow rate of 100 μL min −1 for 30 min and then flushed with sufficient water to wash away the HAuCl 4 residue in bulk.
Selective Fabrication of Au Spiky Nanostars: HAuCl 4 •3H 2 O aqueous solution (5 μL, 0.3 M) was first drop-cast on the substrate covered by asprepared FSGNs.Sixty microliters of 0.1 M HCl was then added into the HAuCl 4 drop and thoroughly mixed with a pipette.Thirty microliters of 160 mM AgNO 3 and 15 μL 1 M AA were simultaneously added into the above droplet and through mixing with a pipette for 5 times and then kept for 30 s. [57] After this, the substrate was rinsed with distilled water at least three times and dried under a gentle N 2 flow.
Dynamics of FSGN Formation and FSGN-Decorated Nanodroplets Interaction with NR: TIRF was performed on a Nikon N-Storm superresolution confocal microscope (TIRF 100 ×, 1.49 NA objective lens).Within the NIS-Elements AR software, the TIRF mirror position was adjusted until achieving TIR, determined by the appearance of interference patterns of the substrate.The region of interest was collected by an Andor iXon DU-897 EMCCD camera, with a pixel calibration of 158 nm per pixel.
For the observation of Au(I)-thiolate complex formation and NR fluorescence change process, the NR (5 μg mL −1 ) was first dissolved in solution A together with 1 vol% dodecanethiol and 1 vol% octanol to produce NR contained binary surface nanodroplets. [60,44]The fluid cell was mounted and illuminated by a 561 nm continuous wave (CW) laser/optical light of the microscope.When the HAuCl 4 solution was injected into the microfluidic chamber, the images of both 561 nm channel and the bright optical field channel were recorded.The fluorescent intensities of each droplet were analyzed as a function of time using home-built Python codes and plotted by Graphpad Prism and Origin software.
Characterization: Optical images and polarized light images of formed surface Au structures were taken using a microscope (Eclipse Ni, Nikon).XPS of samples was characterized using a K-alpha X-Ray photoelectron spectrometer (Thermo Scientific) with the incident radiation using a monochromatic Al K X-rays (1486.7 eV) at 72 W (6 mA and 12 kV).All data were processed using CasaXPS software, and the energy calibration was referenced to the C 1 s peak at 284.8 eV.For SEM observation, the substrate with surface nanostructures was sputter coated by Pt coating (Gatan, Precision Etching and Coating System) with a thickness of ≈5 nm.SEM was performed at high vacuum on FEI VERIOS 460L at 3 kV, 50 pA, and a working distance of 4 mm.

Figure 1 .
Figure 1.Au nanostructure produced by surface nanodroplet approach.a) A schematic diagram of the microfluidic setup and the fabrication of surface Au nanostructures via interfacial reacting HAuCl 4 with binary surface nanodroplets in a microfluidic channel.b) TIRF images and corresponding probability density function (PDF) of dual-component(1%dodecanethiol-1%octanol) surface nanodroplets formed on the glass substrate by solvent exchange process at 400 μL min −1 .Note that the droplets with a radius less than 200 nm were negligible in the quantitative analysis due to a resolution limit of 160 nm per pixel in TIRF.c) Optical images (top) and dark-field images (bottom) of formed surface Au nanostructures.d) SEM images and PDF analysis of formed surface Au nanostructures.A zoom-in image of a representative FSGN was shown as the insert.

Figure 2 .
Figure 2. Dynamics of FSGN formation process.a) Time-dependent TIRF images in the optical channel when binary droplets reacted with AuCl 4 − ions in the flow.The scale bar is 5 μm.b) Schematic illustrations of Au(I)-DT complex formation and their assembly driven by internal flows in a reactive nanodroplet to form a FSGN.The arrows in the left sketch represent generated convection flows.The images are not to scale.

Figure 4 .
Figure 4. Selective generation of Au spiky nanostars on the FSGNs.a) Schematic diagram of the fabrication process and b) SEM images of achieved nanostructures.

Figure 5 .
Figure 5. a) Time-dependent TIRF snapshots of NR encapsulated in binary surface nanodroplets when forming FSGNs.The excitation wavelength was 561 nm.The scale bar is 2 μm.The color bar on the right side represents the degree of fluorescent intensity.b) The relative fluorescent intensity (I/I 0 ) of different-sized surface nanodroplets as a function of time.I 0 is the fluorescent intensity of the surface nanodroplets before injecting AuCl 4 −1 precursors.The fluorescence enhancement along with the FSGN formation is highlighted with a yellow background.c) The pseudo kinetic reaction rate constant K is fitted by Equation 3 under different-sized droplets.The curve is provided as a guide to the eye only.The possible catalytic reaction is presented in the insert.