Light-Induced Polarization-Directed Growth of Optically Printed Gold Nanoparticles

Nanosized-laser-induced sub-20 nm homogenous alloy nanoparticles

Alloy nanoparticles (NPs) have great potential in nanosized 3D-printing, surface coating, plasmonic enhancement, information coding, and so forth. However, chemical-pollution-free and homogeneous sub-20 nm NPs maintain still a challenge in preparation. Here we present a smart nanosecond laser scan strategy of alloy-NPs preparation on a bilayer metal film by using a nanosized focused beam, successfully realizing controllable fabrication of the sub-20 nm homogeneous alloy NPs without pollution. As a demonstration, various sub-20 nm AgCu NPs with different volume ratios have been prepared, all NPs show narrow size distribution and uniform interparticle spacing. This simple and cost-effective method is stable and adaptable for other alloy-NPs such as AuAg NPs. In addition, such alloy NPs exhibit two-peak plasma resonance feature and information coding capacity. We believe that homogenous alloy sub-20 nm NPs will provide new application opportunities in many fields.

Anisotropic growth of Au-Ag heteronanostructures through plasmon-induced reduction

Plasmonic heteronanostructures are promising building blocks for photofunctional materials and devices including photocatalysts, optical materials, and optoelectronic devices. In the present work, we fabricated Au-Ag bimetallic heteronanostructures based on site-selective and anisotropic Ag deposition and growth on Au nanocubes. Plasmonic Au nanocubes were adsorbed onto a glass plate, and the distal mode or proximal-distal mode of the nanocubes was selectively excited in the presence of Ag+ and citrate ions. Polycrystalline Ag was deposited around the top of the Au nanocubes by the distal mode excitation, and single crystalline Ag was grown laterally from the Au nanocubes by the proximal-distal mode excitation. The present method would be applied to the fabrication of various plasmonic nanostructures composed of two or more heterodomains.

Robotic Nanomanipulation Based on Spatiotemporal Modulation of Optical Gradients

Robotic nanomanipulation emerges as a cutting-edge technique pivotal for in situ nanofabrication, advanced sensing, and comprehensive material characterization. In this study, we develop an optical robotic platform (ORP) for the dynamic manipulation of colloidal nanoparticles (NPs). The ORP incorporates a human-in-the-loop control mechanism enhanced by real-time visual feedback. This feature enables the generation of custom optical landscapes with adjustable intensity and phase configurations. Based on the ORP, we achieve the parallel and reconfigurable manipulation of multiple NPs. Through the application of spatiotemporal phase gradient-reversals, our platform demonstrates capabilities in trapping, binding, rotating, and transporting NPs across custom trajectories. This presents a previously unidentified paradigm in the realm of in situ nanomanipulation. Additionally, the ORP facilities a "capture-and-print" assembly process, utilizing a strategic interplay of phase and intensity gradients. This process operates under a constant laser power setting, streamlining the assembly of NPs into any targeted configuration. With its precise positioning and manipulation capabilities, underpinned by the spatiotemporal modulation of optical gradients, the ORP will facilitate the development of colloid-based sensors and on-demand fabrication of nanodevices.

Photothermophoretic Splitting of Gold Nanoparticles for Plasmonic Nanopores and Nanonets Sensing

Optical processing of single plasmonic nanoparticles reinvents the way of high-density information storage, high-performance sensing, and high-definition displays. However, such laser-fabricated nanoplasmonics with well-defined hot spots remain elusive due to the diffraction limit of light. Here we show Au nanoparticle (NP) decorated nanopores can be facilely generated with photothermal splitting of single Au NPs embedded in a silica matrix. The extremely high local temperature induced by plasmonic heating renders gradients of the temperature and surface tension around the Au NP, which drives the nanoscale thermophoretic and Marangoni flow of molten Au/silica. As a result, a nanopore decorated with fragmented Au NPs is formed in the silica film, which presents much stronger surface-enhanced Raman scattering as compared to a single Au NP due to the emergence of hot spots. This strategy can be used to generate plasmonic nanopores of various sizes in the silicon nitride (SiNx) films, which further transforms into nanonets at ambient conditions via light-induced reconstruction of silicon nitride membrane. These nanonets can serve as a robust platform for single particle trapping and analysis.

Thermoplasmonic In Situ Fabrication of Nanohybrid Electrocatalysts over Gas Diffusion Electrodes for Enhanced H2O2 Electrosynthesis

Large-scale development of electrochemical cells is currently hindered by the lack of Earth-abundant electrocatalysts with high catalytic activity, product selectivity, and interfacial mass transfer. Herein, we developed an electrocatalyst fabrication approach which responds to these requirements by irradiating plasmonic titanium nitride (TiN) nanocubes self-assembled on a carbon gas diffusion layer in the presence of polymeric binders. The localized heating produced upon illumination creates unique conditions for the formation of TiN/F-doped carbon hybrids that show up to nearly 20 times the activity of the pristine electrodes. In alkaline conditions, they exhibit enhanced stability, a maximum H2O2 selectivity of 90%, and achieve a H2O2 productivity of 207 mmol gTiN-1 h-1 at 0.2 V vs RHE. A detailed electrochemical investigation with different electrode arrangements demonstrated the key role of nanocomposite formation to achieve high currents. In particular, an increased TiOxNy surface content promoted a higher H2O2 selectivity, and fluorinated nanocarbons imparted good stability to the electrodes due to their superhydrophobic properties.

Polarization-directed growth of spiral nanostructures by laser direct writing with vector beams

Chirality is pivotal in nature which attracts wide research interests from all disciplines and creating chiral matter is one of the central themes for chemists and material scientists. Despite of significant efforts, a simple, cost-effective and general method that can produce different kinds of chiral metamaterials with high regularity and tailorability is still demanding but greatly missing. Here, we introduce polarization-directed growth of spiral nanostructures via vector beams, which is simple, tailorable and generally applicable to both plasmonic and dielectric materials. The self-aligned near field enhances the photochemical growth along the polarization, which is crucial for the oriented growth. The obtained plasmonic chiral nanostructures present prominent optical activity with a g-factor up to 0.4, which can be tuned by adjusting the spirality of the vector beams. These spiral plasmonic nanostructures can be used for the sensing of different chiral enantiomers. The dielectric chiral metasurfaces can also be formed in arrays of sub-mm scale, which exhibit a g-factor over 0.1. However, photoluminescence of chiral cadmium sulfide presents a very weak luminescence g-factor with the excitation of linearly polarized light. A number of applications can be envisioned with these chiral nanostructures such as chiral sensing, chiral separation and chiral information storage.

Site-selective introduction of MnO2 co-catalyst onto gold nanocubes via plasmon-induced charge separation and galvanic replacement for enhanced photocatalysis

For energy harvesting with plasmonic photocatalysis, it is important to optimize geometrical arrangements of plasmonic nanomaterials, electron (or hole) acceptors, and co-catalysts so as to improve the charge separation efficiency and suppress charge recombination. Here we employ a photocatalytic system with Au nanocubes on TiO2, and introduced MnO2 as an oxidation co-catalyst onto the nanocubes via site-selective oxidation based on plasmon-induced charge separation (PICS). However, it has been known that PbO2 is the only material that can be deposited onto Au nanomaterials through PICS with sufficient site-selectivity. Here we addressed this issue by introducing an indirect approach for MnO2 deposition via site-selective PbO2 deposition and subsequent galvanic replacement of PbO2 with MnO2. The indirect approach gave nanostructures with MnO2 introduced at around the top part, bottom part, or the entire surface of the Au nanocubes on a TiO2 electrode. The activity of those plasmonic photocatalysts was strongly dependent on the location of MnO2. The key to improving the activity is to separate MnO2 from TiO2 to prevent recombination of the positive charges in MnO2 with the negative ones in TiO2.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.

Nanoparticle Fragmentation Below the Melting Point Under Single Picosecond Laser Pulse Stimulation

Understanding the laser-nanomaterials interaction including nanomaterial fragmentation has important implications in nanoparticle manufacturing, energy, and biomedical sciences. So far, three mechanisms of laser-induced fragmentation have been recognized including non-thermal processes and thermomechanical force under femtosecond pulses, and the phase transitions under nanosecond pulses. Here we show that single picosecond (ps) laser pulse stimulation leads to anomalous fragmentation of gold nanoparticles that deviates from these three mechanisms. The ps laser fragmentation was weakly dependent on particle size, and it resulted in a bimodal size distribution. Importantly, ps laser stimulation fragmented particles below the whole particle melting point and below the threshold for non-thermal mechanism. We propose a framework based on near-field enhancement and nanoparticle surface melting to account for the ps laser-induced fragmentation observed here. This study reveals a new form of surface ablation that occurs under picosecond laser stimulation at low fluence.

Plasmon-Enhanced Photothermal and Optomechanical Deformations of a Gold Nanoparticle

Plasmon-enhanced photothermal and optomechanical effects on deforming and reshaping a gold nanoparticle (NP) are studied theoretically. A previous paper (Wang and Ding, ACS Nano 13, 32-37, 2019) has shown that a spherical gold nanoparticle (NP) irradiated by a tightly focused laser beam can be deformed into an elongated nanorod (NR) and even chopped in half (a dimer). The mechanism is supposed to be caused by photothermal heating for softening NP associated with optical traction for follow-up deformation. In this paper, our study focuses on deformation induced by Maxwell's stress provided by a linearly polarized Gaussian beam upon the surface of a thermal-softened NP/NR. We use an elastic model to numerically calculate deformation according to optical traction and a viscoelastic model to theoretically estimate the following creep (elongation) as temperature nears the melting point. Our results indicate that a stretching traction at the two ends of the NP/NR causes elongation and a pinching traction at the middle causes a dent. Hence, a bigger NP can be elongated and then cut into two pieces (a dimer) at the dent due to the optomechanical effect. As the continuous heating process induces premelting of NPs, a quasi-liquid layer is formed first and then an outer liquid layer is induced due to reduction of surface energy, which was predicted by previous works of molecular dynamics simulation. Subsequently, we use the Young-Laplace model to investigate the surface tension effect on the following deformation. This study may provide an insight into utilizing the photothermal effect associated with optomechanical manipulation to tailor gold nanostructures.

Plasmon-driven synthesis of individual metal@semiconductor core@shell nanoparticles

Most syntheses of advanced materials require accurate control of the operating temperature. Plasmon resonances in metal nanoparticles generate nanoscale temperature gradients at their surface that can be exploited to control the growth of functional nanomaterials, including bimetallic and core@shell particles. However, in typical ensemble plasmonic experiments these local gradients vanish due to collective heating effects. Here, we demonstrate how localized plasmonic photothermal effects can generate spatially confined nanoreactors by activating, controlling, and spectroscopically following the growth of individual metal@semiconductor core@shell nanoparticles. By tailoring the illumination geometry and the surrounding chemical environment, we demonstrate the conformal growth of semiconducting shells of CeO₂, ZnO, and ZnS, around plasmonic nanoparticles of different morphologies. The shell growth rate scales with the nanoparticle temperature and the process is followed in situ via the inelastic light scattering of the growing nanoparticle. Plasmonic control of chemical reactions can lead to the synthesis of functional nanomaterials otherwise inaccessible with classical colloidal methods, with potential applications in nanolithography, catalysis, energy conversion, and photonic devices.

Plasmon-Induced Photoreduction System Allows Ultrasensitive Detection of Disease Biomarkers by Silver-Mediated Immunoassay

Current strategies for the detection of disease biomarkers often require enzymatic assays that may have limited sensitivity due to inferior stability and vulnerable catalytic activity of the enzyme. A new enzyme-free amplification method for identifying suitable biomarkers is necessary to lower the limit of detection and improve many critical diagnosis applications. Here, we presented an enzyme-free amplified plasmonic immunoassay that enhanced the detection sensitivity of disease biomarkers by combining a novel plasmon-induced silver photoreduction system with a silver nanoparticle (AgNP)-linked immunoassay. The key step to achieving ultrasensitivity was to use Ag⁺ from dissolved AgNPs that control the growth rate of the silver coating on plasmonic nanosensors under visible light illumination. We demonstrated the outstanding sensitivity and robustness of this assay by detecting the disease biomarker alpha-fetoprotein (AFP) at a low concentration of 3.3 fg mL^-1. The detection of AFP was further confirmed in the sera of hepatocellular carcinoma patients.

Seeded Growth of Large-Area Arrays of Substrate Supported Au Nanoparticles Using Citrate and Hydrogen Peroxide

While seeded growth of quasi-spherical colloidal Au nanoparticles (NPs) has been extensively explored in the literature, the growth of surface supported arrays of such particles has received less attention. The latter scenario offers some significant challenges, including the attainment of sufficient particle-substrate adhesion, growth-selectivity, and uniform mass-transport. To this end, a reaction system consisting of HAuCl₄, citrate, and H₂O₂ is here investigated for the growth of supported arrays of 10 nm Au seeds, derived via block copolymer (BCP) lithography. The effects of the reagent concentrations on the properties of the resultant NPs are evaluated. It is found that inclusion of citrate in the growth medium causes substantial particle desorption from Si surfaces. However, the presence of citrate also yields NPs with more uniformly circular top-view cross sections ("quasi-circular"), motivating the exploration of particle immobilization methods. We demonstrate that atomic layer deposition (ALD) of a single cycle of HfO₂ (∼1 Å), after the seed particle formation, promotes adhesion sufficiently to enable the use of citrate without the added oxide noticeably affecting the shape of the resultant NPs. The presented ALD-based approach differs from the conventional sequence of depositing the adhesion layer prior to the seed particle formation and may have advantages in various processing schemes, such as when surface grafting of brush layers is required in the BCP lithography process. A proof-of-concept is provided for the growth of large-area arrays of supported "quasi-circular" Au NPs, in a rapid one-step process at room temperature.

Laser additive nano-manufacturing under ambient conditions

Additive manufacturing at the macroscale has become a hot topic of research in recent years. It has been used by engineers for rapid prototyping and low-volume production. The development of such technologies at the nanoscale, or additive nanomanufacturing, will provide a future path for new nanotechnology applications. In this review article, we introduce several available toolboxes that can be potentially used for additive nanomanufacturing. We especially focus on laser-based additive nanomanufacturing under ambient conditions.

From Optical to Chemical Hot Spots in Plasmonics

In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. In fact, the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons, and molecular states. These degrees of freedom can affect the reaction rates, the product selectivity, or the spatial localization of a chemical reaction. In this Account, we discuss the oportunities to control chemical hot spots by tuning the cascade of events that follows the excitation and decay of plasmonic modes in nanostructures. We discuss a series of techniques to spatially map and image plasmonic nanoscale reactivity at the single photocatalyst level. We show how to optimize the reactivity of carriers by manipulating their excitation and decay mechanisms in plasmonic nanoparticles. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Understanding and controlling these processes is essential for the rational design of solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

Optical Nanoprinting of Colloidal Particles and Functional Structures

Recent advances in chemical sciences have enabled the tailorable synthesis of colloidal particles with variable composition, size, shape, and properties. Building superstructures with colloidal particles as building blocks is appealing for the fabrication of functional metamaterials and nanodevices. Optical nanoprinting provides a versatile platform to print various particles into arbitrary configurations with nanometric precision. In this review, we summarize recent progress in optical nanoprinting of colloidal particles and its related applications. Diverse techniques based on different physical mechanisms, including optical forces, light-controlled electric fields, optothermal effects, laser-directed thermocapillary flows, and photochemical reactions, are discussed in detail. With its flexible and versatile capabilities, optical nanoprinting will find promising applications in numerous fields such as nanophotonics, energy, microelectronics, and nanomedicine.

Spectral Screening of the Energy of Hot Holes over a Particle Plasmon Resonance

Plasmonic hot carriers have been recently identified as key elements for photocatalysis at visible wavelengths. The possibility to transfer energy between metal plasmonic nanoparticles and nearby molecules depends not only on carrier generation and collection efficiencies but also on their energy at the metal-molecule interface. Here an energy screening study was performed by monitoring the aniline electro-polymerization reaction via an illuminated 80 nm gold nanoparticle. Our results show that plasmon excitation reduces the energy required to start the polymerization reaction as much as 0.24 eV. Three possible photocatalytic mechanisms were explored: the enhanced near field of the illuminated particle, the temperature increase at the metal-liquid interface, and the excited electron-hole pairs. This last phenomenon is found to be the one contributing most prominently to the observed energy reduction.

Photothermal-Assisted Optical Stretching of Gold Nanoparticles

The synergy of photothermal energy and optical forces generated by tightly focused laser beams can be used to transform the shape of gold nanoparticles. Here, the combination of these two effects is demonstrated to be an effective way of elongating gold nanoparticles (Au NPs), massively tuning their plasmonic properties. The photothermal effect of the laser increases the temperature of Au NPs above the melting point, and optical forces deform the molten Au NPs. As a result, the shape of Au NPs transforms from nanospheres into nanorods or dimers, depending on the power and time of irradiation as well as the surface energy of the substrate. This process is reversible by using high laser power to transform nanorods back to nanospheres due to capillary dewetting. Such light-induced transformations of nanostructures not only provide a facile way to tune plasmon resonances but also shed light on how the synergistic effect of photothermal energy and optical forces works on plasmonic nanoparticles.

Monitoring plasmonic hot-carrier chemical reactions at the single particle level

Plasmon excitation in metal nanoparticles triggers the generation of highly energetic charge carriers that, when properly manipulated and exploited, can mediate chemical reactions. Single-particle techniques are key to unearthing the underlying mechanisms of hot-carrier generation, transport and injection, as well as to disentangling the role of the temperature increase and the enhanced near-field at the nanoparticle-molecule interface. Gaining nanoscopic insight into these processes and their interplay could aid in the rational design of plasmonic photocatalysts. Here, we present three different approaches to monitor hot-carrier reactivity at the single-particle level. We use a combination of dark-field microscopy and photoelectrochemistry to track a hot-hole driven reaction on a single Au nanoparticle. We image hot-electron reactivity with sub-particle spatial resolution using nanoscopy techniques. Finally, we push the limits by looking for a hot-electron induced chemical reaction that generates a fluorescent product, which should enable imaging plasmonic photocatalysis at the single-particle and single-molecule levels.

Nondestructive Approach for Additive Nanomanufacturing of Metallic Nanostructures in the Air

In this article, a mechanism for quick release and transfer of gold nanoparticles (GNPs) from a soft substrate to another substrate under laser illumination is investigated. The heating of GNPs on a soft substrate with a continuous-wave laser causes a rapid thermal expansion of the substrate, which can be used to selectively release and place GNPs onto another surface. In-plane and out-of-plane nanostructures are successfully fabricated using this method. This rapid release-and-place process can be used for additive nonmanufacturing of metallic nanostructures under ambient conditions, which paves a way for affordable nanomanufacturing and enables a wide variety of applications in nanophotonics, ultrasensitive sensing, and nonlinear plasmonics.

Accuracy and Mechanistic Details of Optical Printing of Single Au and Ag Nanoparticles

Optical printing is a powerful all-optical method that allows the incorporation of colloidal nanoparticles (NPs) onto substrates with nanometric precision. Here, we present a systematic study of the accuracy of optical printing of Au and Ag NPs, using different laser powers and wavelengths. When using light of wavelength tuned to the localized surface plasmon resonance (LSPR) of the NPs, the accuracy improves as the laser power is reduced, whereas for wavelengths off the LSPR, the accuracy is independent of the laser power. Complementary studies of the printing times of the NPs reveal the roles of Brownian and deterministic motion. Calculated trajectories of the NPs, taking into account the interplay between optical forces, electrostatic forces, and Brownian motion, allowed us to rationalize the experimental results and gain a detailed insight into the mechanism of the printing process. A clear framework is laid out for future optimizations of optical printing and optical manipulation of NPs near substrates.

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

In Situ Two-Step Photoreduced SERS Materials for On-Chip Single-Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10^-13 m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.