Chromatographic framework for coffee ring effect-driven separation of small molecules in surface enhanced Raman spectroscopy analysis

The applications of coffee ring effect (CRE) in analytical chemistry have been increasingly expanded from particles and macromolecules to small molecules, in particular coupled to surface-enhanced Raman spectroscopy (SERS). Despite the theory behind the formation of CRE itself from a single drop evaporation onto the dry surface is well established, the theoretical aspects of CRE-driven separation, especially the analyte-surface interactions involving small molecules, have not been conceived. Herein, we have developed a theoretical framework to describe the CRE-driven separation process of small molecules, using SERS analysis of dimethylarsinic acid (DMA^V), dimethylmonothioarsinic acid (DMMTA^V), and dimethyldithioarsinic acid (DMDTA^V) on gold nanofilm (AuNF) as an example. By combining the CRE theory for the radial flow and the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory for mass transfer between solution and AuNF surface, we adapted the conventional chromatographic theory to derive a modified van Deemter equation for the CRE-driven separation. By using this model, we predicted the travel distances of arsenicals based on the different affinity of analytes to the AuNF and evaluated the possibility of separation of unknown analytes by CRE-based SERS, demonstrating the successful adaptation of classic chromatographic theory to CRE-driven nanochromatography.

Physically unclonable functions taggant for universal steganographic prints

Counterfeiting of financial cards and marketable securities is a major social problem globally. Electronic identification and image recognition are common anti-counterfeiting techniques, yet they can be overcome by understanding the corresponding algorithms and analysis methods. The present work describes a physically unclonable functions taggant, in an aqueous-soluble ink, based on surface-enhanced Raman scattering of discrete self-assemblies of Au nanoparticles. Using this stealth nanobeacon, we detected a fingerprint-type Raman spectroscopy signal that we clearly identified even on a business card with a pigment mask such as copper-phthalocyanine printed on it. Accordingly, we have overcome the reverse engineering problem that is otherwise inherent to analogous anti-counterfeiting techniques. One can readily tailor the ink to various information needs and application requirements. Our stealth nanobeacon printing will be particularly useful for steganography and provide a sensitive fingerprint for anti-counterfeiting.

DC field coupled evaporation of a sessile gold nanofluid droplet

The coffee stain formed when a sessile nanofluid colloidal droplet dries on a substrate displays distinct nanoparticle aggregation regimes. We employ scanning electron microscopy to study the coffee stain morphologies when DC electric fields are applied to drying aqueous suspension droplets of CTAB capped gold nanorods (Au-NRs) on a hydrophilic substrate. We observe a typical coffee ring edge with several Au-NR domains due to outward capillary flow both in the absence and presence of the electric field. The Au-NRs at the coffee ring edge assemble in a smectic-like phase with homogeneous alignment in a zero DC field. Despite the presence of strong evaporation-induced flows, application of a DC electric field perpendicular to the substrate results in homeotropic alignment of the Au-NRs at the coffee ring edge. Clusters of Au-NRs with short-range order form at the inner coffee ring edge which we attribute to Marangoni eddies. Moving towards the centre of the coffee stain, we note a depletion region lacking particles, followed by non-uniform deposition of Au-NRs. Au-NR arrays are also found to deposit outside the coffee ring, presumably due to depinning of the evaporating droplet during the initial stages of droplet drying. In contrast to the outer coffee ring edge, we note no change in Au-NR orientation in other regions of the stain due to the extremely low particle concentrations. We believe that our results are applicable to assemblies of a variety of surfactant capped metal nanorods.

Glycerol-Assisted Construction of Long-Life Three-Dimensional Surface-Enhanced Raman Scattering Hot Spot Matrix

An evaporative self-assembly strategy for constructing a long-life 3D surface-enhanced Raman scattering (SERS) hot spot matrix is proposed with the assistance of glycerol to improve the spectral sensitivity and reproducibility. Different from the traditional wetting-state or drying-state methods, silver nanoparticles (AgNPs) in the glycerol-stabilized 3D SERS hot spot matrix can be maintained in the translation state for more than 7 days with the maximal uniformity of the interparticle distance. Brownian dynamics simulations reveal that more hot spots emerge in the glycerol-stabilized AgNPs matrix, and the distances between the AgNPs are not fixed but balanced in a small range by the interplay of the van der Waals attraction and the electrostatic repulsion. A 2 orders of magnitude extra SERS enhancement, more stable peak frequencies, and a narrower peak full width at half-maximum (fwhm) can therefore be obtained, which ensure extremely stable and reproducible SERS signals. Single-molecule detection sensitivity utilizing the glycerol-stabilized 3D SERS hot spot matrix has been demonstrated by collecting the SERS spectra of dye molecules at a concentration of as low as 0.5 aM (5 × 10^-19 M) with a good signal-to-noise ratio. A long lifetime, ultrahigh SERS enhancement, and extremely stable peak shape make the 3D SERS hot spot matrix a sensitive, practical, and reliable tool for the detection and analysis of analytes at ultralow concentration or even at the single-molecule level in complex matrixes.

Suppression of coffee-ring effect via periodic oscillation of substrate for ultra-sensitive enrichment towards surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) has attracted extensive interest due to excellent molecule recognition and sensitive concentration detection. Nevertheless, the coffee ring effect (CR) during the analyte evaporation always causes an uneven distribution of the assembled hot-spots, and hence the unreliable SERS signal is produced. In this study, for the first time, we present a suppressed coffee ring (SCR) system via a combination of a magnetically functionalized membrane and reciprocating magnetic field to dynamically suppress the CR for highly reliable and ultra-sensitive SERS detection. The enrichment mechanism of the nanoparticles and the analyte molecules within the sessile droplet based on the proposed system was studied. We experimentally observed that the driving frequency could well affect the final pattern, and typically a higher driving frequency facilitated a smaller coverage area with better enrichment performance. With the use of R6G molecule and (100 nm) gold nanoparticles, we examined the uniformity and SERS of the assembled 'hot-spots' in the SCR system. The results indicate that the uniformity can be greatly improved via SCR in comparison of ring stain, with the RSD of a Raman signal as low as 7.1% even at a low concentration of 10^-12 mol L^-1. Such system also enables the further enhancement in the SERS signal, with the detection limit down to 10^-16 mol L^-1, the enhancement factor magnitude up to 10¹³, and the linear relationship between the SERS intensity and the analyte concentrations within the range of 10^-6-10^-12 and 10^-12-10^-16 mol L^-1, respectively. The applicability of the SCR-based SERS detection for diverse analytes was also proved with a similar but further enhanced signal of MB and 4-ATP. We believe that the excellent SCR-based SERS performance via the proposed system has great potentials for ultra-sensitive detection and/or precise quantitative analysis in various research fields and applications.

Controllability of cupric particle synthesis by linear alcohol chain number as additive and pH control in cupric acetate solution using X-ray radiolysis

Synthesis and immobilization of caltrop cupric particles onto a Si substrate using X-ray radiolysis directly from a liquid solution of Cu(COOCH₃)₂ is demonstrated. Caltrop cupric oxide particles are formed in the X-ray radiolysis of aqueous solutions of Cu(COOCH₃)₂, which also contain methanol, ethanol, 2-propanol or 1-propanol as ^\bulletOH scavenger. The blade lengths of the caltrop particles are dependent on the alcohol chain length. In particular, it was found that an alkyl alcohol whose chain length is longer than four is unable to synthesize any particles in aqueous solutions of Cu(COOCH₃)₂ in X-ray radiolysis. These results are attributed to the alkyl alcohol chain length influencing the rate of reaction of radicals and determines the solvable ratio of its alcohol into water. In addition, it was found that the synthesized particle geometric structure and composition can also be controlled by the pH of the aqueous solution in the X-ray radiolysis. This study may open a door to understanding and investigating a novel photochemical reaction route induced under X-ray irradiation. The development of the X-ray radiolysis process enables us to achieve the rapid and easy process of synthesis and immobilization of higher-order nano/microstructure consisting of various materials.

Photovoltaic cells as a highly efficient system for biomedical and electrochemical surface-enhanced Raman spectroscopy analysis

Surface-enhanced Raman scattering (SERS) has been intensively used recently as a highly sensitive, non-destructive, chemical specific, and label-free technique for a variety of studies. Here, we present a novel SERS substrate for: (i) the standard ultra-trace analysis, (ii) detection of whole microorganisms, and (iii) spectroelectrochemical measurements. The integration of electrochemistry and SERS spectroscopy is a powerful approach for in situ investigation of the structural changes of adsorbed molecules, their redox properties, and for studying the intermediates of the reactions. We have developed a conductive SERS platform based on photovoltaic materials (PV) covered with a thin layer of silver, especially useful in electrochemical SERS analysis. These substrates named Ag/PV presented in this study combine crucial spectroscopic features such as high sensitivity, reproducibility, specificity, and chemical/physical stability. The designed substrates permit the label-free identification and differentiation of cancer cells (renal carcinoma) and pathogens (Escherichia coli and Bacillus subtilis). In addition, the developed SERS platform was adopted as the working electrode in an electrochemical SERS approach for p-aminothiophenol (p-ATP) studies. The capability to monitor in real-time the electrochemical changes spectro-electro-chemically has great potential for broadening the application of SERS.

Caltrop particles synthesized by photochemical reaction induced by X-ray radiolysis

X-ray radiolysis of a Cu(CH₃COO)₂ solution was observed to produce caltrop-shaped particles of cupric oxide (CuO, Cu₂O), which were characterized using high-resolution scanning electron microscopy and micro-Raman spectrometry. X-ray irradiation from a synchrotron source drove the room-temperature synthesis of submicrometer- and micrometer-scale cupric oxide caltrop particles from an aqueous Cu(CH₃COO)₂ solution spiked with ethanol. The size of the caltrop particles depended on the ratio of ethanol in the stock solution and the surface of the substrate. The results indicated that there were several synthetic routes to obtain caltrop particles, each associated with electron donation. The technique of X-ray irradiation enables the rapid synthesis of caltrop cupric oxide particles compared with conventional synthetic methods.

Colloidal Drop Deposition on Porous Substrates: Competition among Particle Motion, Evaporation, and Infiltration

Recent interest in printable electronics and in particular paper- and textile-based electronics has fueled research in inkjet printing of colloidal drops on porous substrates. On nonporous substrates, the interplay of particle motion and solvent evaporation determines the final deposition morphology of the evaporating colloidal drop. For porous substrates, solvent infiltration into the pores adds a layer of complexity to the deposition patterns that have not been fully elucidated in the literature. In this study, the deposition of picoliter-sized aqueous colloidal droplets containing nanometer- and micrometer-sized particles onto nanoporous anodic aluminum oxide substrates is examined for different drop and particle sizes and relative humidities as well as pore diameters, porosities, and wettabilities of the porous substrates. For the cases considered, solvent infiltration is found to be much faster than both evaporation and particle motion near the contact line, and thus when the substrate fully imbibes the solvent, the well-known "coffee-ring" deposition is suppressed. However, when the solvent is only partially imbibed, a residual droplet volume exists upon completion of the infiltration. For such cases, two time scales are of importance: the time for particle motion to the contact line as a result of both diffusion and advection, t(P), and the evaporation time of the residual drop volume, t(EI). Their ratio, t(P)/t(EI), determines whether the coffee-ring deposition will be formed (t(P)/t(EI) < 1) or suppressed (t(P)/t(EI) > 1).

Anisotropic optical properties of large-scale aligned silver nanowire films via controlled coffee ring effects

Preparation of thin films with one-dimensional nanostructures and unique physical properties for high-performance electronic, optoelectronic, and electromechanical systems.

On-chip integration of novel Au electrode with a higher order three-dimensional layer stack nanostructure for surface-enhanced Raman spectroscopy

We develop a novel in situ surface-enhanced Raman spectroscopy (SERS) platform with three-dimensional nanostructure gold electrodes using the competitive self-assembly between dielectrophoresis and convective aggregation.