Trace Analysis and Chemical Identification on Cellulose Nanofibers-Textured SERS Substrates Using the "Coffee Ring" Effect

Electrochemical synthesis of 2D-silver nanodendrites functionalized with cyclodextrin for SERS-based detection of herbicide MCPA

Surface enhanced Raman spectroscopy (SERS) is a powerful analytical technique that has found application in the trace detection of a wide range of contaminants. In this paper, we report on the fabrication of 2D silver nanodendrites, on silicon chips, synthesized by electrochemical reduction of AgNO3at microelectrodes. The formation of nanodendrites is tentatively explained in terms of electromigration and diffusion of silver ions. Electrochemical characterization suggests that the nanodendrites do not stay electrically connected to the microelectrode. The substrates show SERS activity with an enhancement factor on the order of 106. Density functional theory simulations were carried out to investigate the suitability of the fabricated substrate for pesticide monitoring. These substrates can be functionalized with cyclodextrin macro molecules to help with the detection of molecules with low affinity with silver surfaces. A proof of concept is demonstrated with the detection of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA).

Cellulose Nanofiber Films with Gold Nanoparticles Electrostatically Adsorbed for Facile Surface-Enhanced Raman Scattering Detection

Cellulose nanofiber (CNF) holds great promise in applications such as surface-enhanced Raman scattering (SERS), catalysis, esthesia, and detection. This study aimed to build novel CNF-based SERS substrates through a facile synthetic method. Citrate-reduced gold nanoparticles (AuNPs) were adsorbed on the cationized CNF surface due to electrostatic interactions, and uniform AuNPs@(2,3-epoxypropyl trimethylammonium chloride)EPTMAC@CNF flexible SERS substrates were prepared by a simple vacuum-assisted filtration method. The probe molecule methylene blue was chosen to assess the performance of the CNF-based SERS substrate with a sensitivity up to 10-9 M, superior signal reproducibility (relative standard deviation (RSD) = 4.67%), and storage stability (more than 30 days). Tensile strength tests indicated that the CNF-based films had good mechanical properties. In addition, CNF-based substrates can easily capture and visually identify microplastics in water. These results demonstrate the potential application of the flexible, self-assembled AuNPs@EPTMAC@CNF flexible SERS substrate for prompt and sensitive detection of trace substances.

Levofloxacin loaded poly (ethylene oxide)-chitosan/quercetin loaded poly (D,L-lactide-co-glycolide) core-shell electrospun nanofibers for burn wound healing

This study developed a new burn wound dressing based on core-shell nanofibers that co-deliver antibiotic and antioxidant drugs. For this purpose, poly(ethylene oxide) (PEO)-chitosan (CS)/poly(D,L-lactide-co-glycolide) (PLGA) core-shell nanofibers were fabricated through co-axial electrospinning technique. Antibiotic levofloxacin (LEV) and antioxidant quercetin (QS) were incorporated into the core and shell parts of PEO-CS/PLGA nanofibers, respectively. The drugs could bond to the polymer chains through hydrogen bonding, leading to their steady release for 168 h. An in vitro drug release study showed a burst effect followed by sustained release of LEV and QS from the nanofibers due to the Fickian diffusion. The NIH 3T3 fibroblast cell viability of the drug loaded core-shell nanofibers was comparable to that in the control (tissue culture polystyrene) implying biocompatibility of the nanofibers and their cell supportive role. However, there was no significant difference in cell viability between the drug loaded and drug free core-shell nanofibers. According to in vivo experiments, PEO-CS-LEV/PLGA-QS core-shell nanofibers could accelerate the healing process of a burn wound compared to a sterile gauze. Thanks to the synergistic therapeutic effect of LEV and QS, a significantly higher wound closure rate was recorded for the drug loaded core-shell nanofibrous dressing than the drug free nanofibers and control. Conclusively, PEO-CS-LEV/PLGA-QS core-shell nanofibers were shown to be a promising wound healing material that could drive the healing cascade through local co-delivery of LEV and QS to burn wounds.

Application of Nanohybrid Substrates with Layer-by-Layer Self-Assembling Properties to High-Sensitivity Surface-Enhanced Raman Scattering Detection

The present study was conducted to prepare and investigate large-area, high-sensitivity surface-enhanced Raman scattering (SERS) substrates. Organic/inorganic nanohybrid dispersants consisting of an amphiphilic triblock copolymer (hereafter referred to simply as "copolymer") and graphene oxide (GO) were used to stabilize the growth and size of gold nanoparticles (AuNPs). Ion-dipole forces were present between the AuNPs and copolymer dispersants, while the hydrogen bonds between GO and the copolymer prevented the aggregation of GO, thereby stabilizing the AuNP/GO nanohybrids. Transmission electron microscopy (TEM) revealed that the AuNPs had particle sizes of 25-35 nm and a relatively uniform size distribution. The AuNP/GO nanohybrids were deposited onto the glass substrate by using the solution drop-casting method and employed for SERS detection. The self-assembling properties of two-dimensional sheet-like GO led to a regular lamellar arrangement of AuNP/GO nanohybrids, which could be used for the preparation of large-area SERS substrates. Following removal of the copolymer by annealing at 300 °C for 2 h, measurements were obtained under scanning electron microscopy. The results confirmed that 2D GO nanosheets were capable of stabilizing AuNPs, with the final size reaching approximately 40 nm. These AuNPs were adsorbed on both sides of the GO nanosheets. Because the GO nanosheets were merely 5 nm-thick, a good three-dimensional hot-junction effect was generated along the z-axis of the AuNPs. Lastly, the prepared material was used for the SERS detection of rhodamine 6G (R6G), a commonly used highly fluorescent dye. An enhancement factor (EF) of up to 3.5 × 106 was achieved, and the limit of detection was approximately 10-10 M. Detection limits of 10-10 M and < 10-10 M were also observed with the detection of Direct Blue 200 and the biological molecule adenine. It is therefore evident that AuNP/copolymer/GO nanohybrids are large-area flexible SERS substrates that hold great potential in environmental monitoring and biological system detection applications.

Determination of thiram in fruit juices using a bacterial cellulose nanocrystal-based SERS substrate

Pesticide residues in agri-foods have risk to human health and one solution is to develop simple and accurate methods for rapid detection. We developed a SERS sensor composed of gold nanoparticles (AuNPs) and bacterial cellulose nanocrystal (BCNC) to detect thiram in fruit juice. BCNC-SO3H was used as a stabilizer to support AuNPs via electrostatic repulsion, fabricating a BCNC-AuNPs SERS substrate with uniformly distributed AuNPs. This BCNC-AuNPs SERS substrate was applied to determine thiram residues in peach juice, apple juice, and grape juice with the limits of detection of 0.036 ppm, 0.044 ppm, and 0.044 ppm, respectively. The whole test took 12 min including sample preparation and analysis. The detection limits meet the maximum residue levels of thiram in fruit juices required by China, Europe and North America, indicating that this BCNC-based substrate could serve as a satisfactory SERS sensor for pesticide residue monitoring in the food supply chain.

Development of a Low-Cost Paper-Based Platform for Coffee Ring-Assisted SERS

The need for highly sensitive, low-cost, and timely diagnostic technologies at the point of care is increasing. Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique that is an advantageous technique to address this need, as it can rapidly detect analytes in small or dilute samples with improved sensitivity compared to conventional Raman spectroscopy. Despite the many advantages of SERS, one drawback of the technique is poor reproducibility due to variable interactions between nanoparticles and target analytes. To overcome this limitation, coupling SERS with the coffee ring effect has been implemented to concentrate and localize analyte-nanoparticle conjugates for improved signal reproducibility. However, current coffee ring platforms require laborious fabrication steps. Herein, we present a low-cost, two-step fabrication process for coffee ring-assisted SERS, utilizing wax-printed nitrocellulose paper. The platform was designed to produce a highly hydrophobic paper substrate that supports the coffee ring effect and tested using gold nanoparticles for SERS sensing. The nanoparticle concentration and solvent were varied to determine the effect of solution composition on ring formation and center clearance. The SERS signal was validated using 4-mercaptobenzoic acid (MBA) and tested with Moraxella catarrhalis bacteria to ensure functionality for chemical and biological applications. The limit of detection using MBA is 41.56 nM, and the biochemical components of the bacterial cell wall were enhanced with low spectral variability. The developed platform is advantageous due to ease of fabrication and use, representing the next step toward implementing low-cost coffee ring-assisted SERS for point-of-care sensing.

Single versus Double Coffee-Ring Effect Patterns in Thin-Layer Chromatography Coupled with Surface-Enhanced Raman Spectroscopic Analysis of Anti-Diabetic Drugs Adulterated in Herbal Products

In thin-layer chromatography coupled with surface-enhanced Raman spectroscopy (TLC-SERS), the coffee ring effect (CRE) describes the formation of a ring-shape spot (blank in the middle and darker on the edge) caused by the aggregation of silver nanoparticles (Ag NPs), alone (single CRE) or with the analytes (double CRE). In this work, the SCRE and DCRE were investigated in two anti-diabetic drugs, hydrophobic glibenclamide (GLB) and more hydrophilic metformin (MET). The SCRE occurred in GLB analysis, as opposed to the DCRE that occurred in MET. It was proven that for optimization of the TLC-SERS analytical procedure, it is necessary to distinguish the CRE patterns of analytes. Additionally, MET and GLB were analyzed with the developed TLC-SERS method and confirmed by another validated method using high-performance liquid chromatography. Four herbal products collected on the market were found to be adulterated with GLB or/and MET; among those, one product was adulterated with both MET and GLB, and two products were adulterated with GLB at a higher concentration than the usual GLB prescription dose. The TLC-SERS method provided a useful tool for the simultaneous detection of adulterated anti-diabetic herbal products, and the comparison of the SCRE and DCRE provided more evidence to predict CRE patterns in TLC-SERS.

Stretchable and Flexible Micro-Nano Substrates for SERS Detection of Organic Dyes

Surface-enhanced Raman spectroscopy (SERS) is a precise and noninvasive analytical technique to identify vibrational fingerprints of trace analytes with sensitivity down to the single-molecule level. However, substrates can influence this capability, and current SERS techniques lack uniform, reproducible, and stable substrates to control plasma hot spots over a wide spectral range. Herein, we demonstrate a flexible SERS substrate via longitudinal stretching of a polydimethylsiloxane (PDMS) film. This substrate, after stretching and shrinking, exhibits an irregular wrinkled structure with abundant gaps and grooves that function as hot spots, thereby improving the hydrophobic properties of the material. To investigate the enhancement effect of Raman signals, silver nanoparticles (AgNPs) were mixed with Rhodamine 6G (R6G) solution, and the obtained blend was dropped onto the PDMS film to form a coffee ring pattern. According to the results, the hydrophobicity of the substrate increases with the degree of PDMS stretching, achieving the optimal level at 150% stretching. Moreover, the increase in hydrophobicity makes the measured molecules more aggregated, which enhances the Raman signal. The stretching and shrinkage of the PDMS film lead to a much higher density of nanogaps among nanoparticles and nanogrooves, which serve as multiple hot spots. Being highly localized regions of intense local fields, these hot spots make a significant contribution to SERS performance, improving the sensitivity and reproducibility of the method. In particular, the relative standard deviation (RSD) was found to be 2.5544%, and the detection limit was 1 × 10^-7 M. Therefore, SERS using stretchable and flexible micro-nano substrates is a promising way for detecting dyes in wastewater.

Ni and O co-modified MoS2 as universal SERS substrate for the detection of different kinds of substances

Surface-enhanced Raman scattering (SERS) has attracted extensive attention as an ultrasensitive detection method. However, the poor biocompatibility and expensive synthesis cost of noble metal SERS substrates have become non-negligible factors that limit the development of SERS technology. Metal chalcogenide semiconductors as an alternative to noble metal SERS substrates can avoid these disadvantages, but the enhancement effect is lower than that of noble metal substrates. Here, we report a method to co-modify MoS₂ by Ni and O, which improves the carrier concentration and mobility of MoS₂. The SERS effect of the modified MoS₂ is comparable to that of noble metals. We found that the improved SERS performance of MoS₂ can be attributed to the following two factors: strong interfacial dipole-dipole interaction and efficient charge transfer effect. During the doping process, the incorporation of Ni and O enhances the polarity and carrier concentration of MoS₂, enhances the interfacial interaction of MoS₂, and provides a basis for charge transfer. During the annealing process, the introduction of O atoms into the S defects reduces the internal defects of doped MoS₂, improves the carrier mobility, and promotes the efficient charge transfer effect of MoS₂. The final modified MoS₂ as a SERS substrate realizes low-concentration detection of bilirubin, cytochrome C, and trichlorfon. This provides promising guidance for the practical inspection of metal chalcogenide semiconductor substrates.

Quantitative Label-Free SERS Detection of Trace Fentanyl in Biofluids with a Freestanding Hydrophobic Plasmonic Paper Biosensor

The prevalence of fentanyl abuse raises global public health concerns with an unprecedented surge in overdose deaths. Rapid identification and quantification of fentanyl in biofluids is of paramount importance to combat fentanyl abuse for law enforcement agencies and promptly treat patients for medical professionals. Herein, a freestanding surface-enhanced Raman spectroscopy (SERS) biosensor with excellent condensing enrichment capability, termed FrEnSERS biosensor, is reported for quantitative label-free detection of trace fentanyl in biofluids. This biosensor comprises a reduced graphene oxide membrane decorated with high-density hydrophobic Au nanostars. A combination of the high SERS enhancement and the focusing effect for analyte enrichment of the hydrophobic surface accounts for the remarkable SERS performance of the FrEnSERS biosensor. We demonstrate that the FrEnSERS biosensor achieves the sensitive and quantitative detection of fentanyl in both serum and urine over a wide dynamic range spanning more than 4 orders of magnitude, with a limit of detection of 0.47 ng/mL for serum samples and 0.73 ng/mL for urine samples. Our biosensor is sensitive, cost-effective, and reliable for rapid quantitative analysis of fentanyl in biofluids with great promise for forensic analysis and clinical diagnosis.

A Rapid Therapeutic Drug Monitoring Strategy of Carbamazepine in Serum by Using Coffee-Ring Effect Assisted Surface-Enhanced Raman Spectroscopy

Carbamazepine (CBZ) has a narrow therapeutic concentration range, and therapeutic drug monitoring (TDM) is necessary for its safe and effective individualized medication. This study aims to develop a procedure for CBZ detection in serum using coffee-ring effect assisted surface-enhanced Raman spectroscopy (SERS). Silver nanoparticles deposited onto silicon wafers were used as the SERS-active material. Surface treatment optimization of the silicon wafers and the liquid-liquid extraction method were conducted to eliminate the influence of impurities on the silicon wafer surface and the protein matrix. The proposed detection procedure allows for the fast determination of CBZ in artificially spiked serum samples within a concentration range of 2.5-40 μg·mL^-1, which matches the range of the drug concentrations in the serum after oral medication. The limit of detection for CBZ was found to be 0.01 μg·mL^-1. The developed method allowed CBZ and its metabolites to be ultimately distinguished from real serum samples. The developed method is anticipated to be a potential tool for monitoring other drug concentrations.

Hydroxylamine mediated Fenton-like interfacial reaction dynamics on sea urchin-like catalyst derived from spent LiFePO4 battery

Herein, we converted spent LiFePO₄ battery to the sea urchin-like material (SULM) with a highly efficient and environment-friendly method, which can contribute to building a zero-waste city. With SULM as a Fenton-like catalyst, a highly-efficient degradation process was realized for organic pollutants with interface and solution synergistic effect. In our SULM+NH₂OH+H₂O₂ Fenton-like system, NH₂OH can effectively promote the interface iron (Fe(Ⅲ)/Fe(Ⅱ)) and solution iron (Fe(Ⅲ)/Fe(Ⅱ)) redox cycle, thus promoting the generation of reactive oxygen species (ROS). However, the ROS generation process and organic pollutants degradation pathway with the presence of NH₂OH remains a puzzle. Here the detailed ROS generation mechanism and pollutants degradation pathway have been illustrated carefully based on experimental exploration and characterization. Therein, hydroxyl radicals (·OH) and singlet oxygen (¹O₂) are the main ROS for oxidizing and degrading organic pollutants. Notably, ¹O₂ can be converted from superoxide radicals (·O₂) in SULM+NH₂OH+H₂O₂ system. This study not only demonstrates the strategy of "trash-to-treasure" and "waste-to-control-waste" to simultaneously reduce the hazardous release from industrial solid waste and organic wastewater, it also provides new mechanistic insights for NH₂OH mediated Fenton-like redox system.

Surface-Enhanced Raman Spectroscopy Substrates for Food Safety and Quality Analysis

Surface-enhanced Raman spectroscopy (SERS) has been identified as a fundamental surface-sensitive technique that boosts Raman scattering by adsorbing target molecules on specific surfaces. The application of SERS highly relies on the development of smart SERS substrates, and thus the fabrication of SERS substrates has been constantly improved. Herein, we investigate the impacts of different substrates on SERS technology including plasmonic metal nanoparticles, semiconductors, and hybrid systems in quantitative food safety and quality analysis. We first discuss the fundamentals, substrate designs, and applications of SERS. We then provide a critical review of the recent progress of SERS in its usage for screening and detecting chemical and biological contaminants including fungicides, herbicides, insecticides, hazardous colorants, and biohazards in food samples to assess the analytical capabilities of this technology. Finally, we investigate the future trends and provide practical techniques that could be used to fulfill the requirements for rapid analysis of food at a low cost.

Development of Gold Nanoparticle-Based SERS Substrates on TiO2-Coating to Reduce the Coffee Ring Effect

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO₂-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)₄ substrates and calcinated as well as plasma treated TiO₂ substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

Label-free surface enhanced Raman spectroscopy analysis of blood serum via coffee ring effect for accurate diagnosis of cancers

Surface-enhanced Raman spectroscopy (SERS) is considered as an ultrasensitive, non-invasive as well as rapid detection technology for cancer diagnosis. In this study, we developed a novel blood serum analysis strategy using coffee ring effect-assisted label-free SERS for different types of cancer screening. Additionally, the pretreated Ag nanoparticles (Ag NPs) were mixed with the serum from liver cancer patients (n = 40), prostate cancer patients (n = 32) and healthy volunteers (n = 30) for SERS measurement. The droplets of Ag NPs-serum mixture formed the coffee ring on the peripheral after air-drying, and thus extremely enhancing Raman signal and ensuring the stability and reliability of SERS detection. Partial least square (PLS) and support vector machine (SVM) algorithms were utilized to establish the diagnosis model for SERS spectra data classifying, yielding the high diagnostic accuracy of 98.04% for normal group and two types of cancers simultaneously distinguishing. More importantly, for the unknown testing set, an ideal diagnostic accuracy of 100% could be achieved by PLS-SVM algorithm for differentiating cancers from the normal group. The results from this exploratory work demonstrate that serum SERS detection combined with PLS-SVM diagnostic algorithm and coffee ring effect has great potential for the noninvasive and label-free detection of cancer.

Extraordinary approach to further boost plasmonic NIR-SERS by cryogenic temperature-suppressed non-radiative recombination

We report an effective strategy to promote the near-infrared surface-enhanced Raman scattering spectroscopy (NIR-SERS) activity by boosting the photon-induced charge transfer (PICT) efficiency at cryogenic temperature. Based on as-prepared Au/Ag nano-urchins (NUs) with abundant surface defects, the extremely low temperature (77 K) can significantly weaken the metallic lattice vibration and reduce the recombination of thermal phonons and photoexcited electrons, then accelerate the migration of energetic electrons. It enables the NIR-SERS detection limit of dye molecules to be achieved at 10^-17 M, which is nearly three orders of magnitude better than that at room temperature. The present work provides a new, to the best of our knowledge, approach for ultra-trace NIR-SERS bioanalysis.

Nylon membranes modified by gold nanoparticles as surface-enhanced Raman spectroscopy substrates for several pesticides detection

Surface enhanced Raman spectroscopy (SERS) is an attractive means for trace compound detection because of its high sensitivity, however, the poor reproducibility is a major challenge. Herein, we propose a facile SERS strategy employing the several developed test processes to improve the repeatability of the SERS analysis based on regular nylon membranes as substrates to detect trace compounds. Various methods, including in situ reduction, immersion adsorption, and filtration, were first compared to prepare composite substrates using nylon membranes and gold nanoparticles. The substrates prepared by filtration showed the best test parallelism (RSD = 7.85%). Its limit of detection (LOD) could reach 10^-8 g mL^-1 with a good linear relationship in the range 10^-8 to 10^-7 g mL^-1. Finally, three pesticide solutions were tested to verify the substrate applicability. A superior LOD of 10^-8 g mL^-1 was observed for thiram, whereas the LODs of both phorate and benthiocarb could reach 10^-6 g mL^-1. Overall, modifying nylon membrane substrates with gold nanoparticles improves the repeatability and economic viability of SERS and favors its wider commercial application for detecting trace compounds.

Surface-Enhanced Raman Scattering-Active Plasmonic Metal Nanoparticle-Persistent Luminescence Material Composite Films for Multiple Illegal Dye Detection

Uniform two-dimensional plasmonic nanoparticle (NP)-semiconductor composite films could retard the attenuation of electromagnetic evanescent wave and show intensive Raman activity for the multiplex monitoring of hazards in a practical food matrix. Here, an efficient Raman platform is developed by employing a plasmonic nanoparticle (NP)-persistent luminescence material (PLM) composite film. PLM show upconversion photoluminescence (UCPL) properties. The emitted photons are absorbed by plasmonic NPs, which further boost the surface plasmon resonance for the generation of high polarizability and induce strong electromagnetic strength for surface-enhanced Raman scattering (SERS) enhancement. A UCPL-assisted SERS-enhanced mechanism is proposed and verified. A plasmonic NP-PLM film with superior SERS activity and detection capability becomes an alternative candidate for the sensitive and multiple detection of illegal addition of dyes in a food matrix. The proposed UCPL-assisted SERS-enhanced mechanism provides promising future directions to this end to design a next-generation SERS-active plasmonic NP-PLM composite film for the specific detection in complex samples.

Dual-Purpose SERS Sensor for Selective Determination of Polycyclic Aromatic Compounds via Electron Donor-Acceptor Traps

Toxic, carcinogenic, and mutagenic properties of polycyclic aromatic hydrocarbons (PAHs) and environmental pollution caused by polycyclic aromatic sulfur heterocycles (PASHs) postulate the importance of their selective and sensitive determination in environmental and oil fuel samples. Surface-enhanced Raman spectroscopy (SERS) opens up an avenue toward multiplex analysis of complex mixtures, however not every molecule gives high enhancement factors and, thus, cannot be reliably detected via SERS. However, the sensitivity can be drastically increased by additional resonant enhancement as a result of the analyte absorption band overlapping with the surface plasmon band of nanoparticles (NPs) and the laser excitation wavelength. Using this idea, we developed a dual-purpose SERS sensor based on trapping the target PAHs and PASHs into colored charge-transfer complexes (CTCs) with selected organic π-acceptor molecules on the surface of AgNPs. Studying, computing, and then comparing stability constants of the formed CTC served as a powerful explanation and prediction tool for a wise choice of π-acceptor indicator systems for the further silver surface modification. Moreover, we show that CTC formation can be effectively utilized for increasing both selectivity and sensitivity by simple liquid-liquid extraction prior to SERS measurements. For the first time, the dual-purpose SERS sensor allowed determination of two different classes of polycyclic aromatic fuel components down to 10 nM concentration, lower than that restricted by the ASTM regulation, and demonstrated multi-purpose capabilities of the developed approach.

Immobilization of Cubic Silver Plasmonic Nanoparticles on TiO2 Nanotubes, Reducing the Coffee Ring Effect in Surface-Enhanced Raman Spectroscopy Applications

Surface-enhanced Raman spectroscopy (SERS) substrates prepared by immobilizing silver cubic nanoparticles (Ag CNPs) on titanium dioxide nanotubes (TiO₂ NTs) were used for investigations of the "coffee ring" (CR) effect and its impact on spatial reproducibility of measured Raman signals in comparison with flat surfaces (Ti and Si) where the CR effect is usually significant. The immobilization of nanoparticles from drops, which is a very simple technique, usually does not permit a homogeneous distribution of deposited NPs because there is significant accumulation of the material at the boundary of the drying area. Our proposed SERS substrates effectively reduced the CR effect through the use of well-ordered nanostructures where a smaller number of Ag CNPs were transferred to the boundary region. It was not only the surface morphology that was important but also the physicochemical properties of TiO₂ NTs, such as wettability. The wettability of the prepared samples was determined by measuring the static water contact angle (WCA), and the chemical composition near the boundary of the drying area was studied using Auger electron spectroscopy. The morphology of the substrates obtained was characterized using scanning electron microscopy. Our studies showed that reducing the coffee ring effect increased the spatial reproducibility of the measured SERS signal in the area of the deposited CNPs. Therefore, the platforms obtained may be very useful in commercial SERS applications.

Reproducible and Bendable SERS Substrates with Tailored Wettability Using Block Copolymers and Anodic Aluminum Oxide Templates

Surface properties are essential for substrates exhibiting high sensitivity in surface-enhanced Raman scattering (SERS) applications. In this work, novel SERS hybrid substrates using polystyrene-block-poly(methyl methacrylate) and anodic aluminum oxide templates is presented. The hybrid substrates not only possess hierarchical porous nanostructures but also exhibit superhydrophilic surface properties with the water contact angle ≈0°. Such surfaces play an important role in providing uniform enhanced intensities over large areas (relative standard deviation ≈10%); moreover, these substrates are found to be highly sensitive (limit of detection ≈10^-12 m for rhodamine 6G (R6G)). The results show that the hybrid SERS substrates can achieve the simultaneous detection of multicomponent mixtures of different target molecules, such as R6G, crystal violet, and methylene blue. Furthermore, the bending experiments show that about 70% of the SERS intensities are maintained after bending from ≈30° to 150°.

Carboxylic Acid-Functionalized, Graphitic Layer-Coated Three-Dimensional SERS Substrate for Label-Free Analysis of Alzheimer's Disease Biomarkers

Surface-enhanced Raman spectroscopy (SERS)-based protein analysis is a promising alternative to existing early stage diagnoses. However, SERS research conducted thus far accompanies challenges such as nonuniformity of plasmonic nanostructures, irregular coating of analytes, and denaturation of proteins, which seriously limit the practicability of suggested approaches. Here, we introduce a carboxylic acid-functionalized and graphitic nanolayer-coated three-dimensional SERS substrate (CGSS) fabricated by sequential nanotransfer printing. The substrate consists of well-defined, uniform gold nanowire arrays for effective Raman signal enhancement and a strong protein-immobilization layer. With an enhancement factor (EF) of 5.5 × 10⁵, on par with the highest ever reported values, the CGSS allows the detection of protein conformational changes and the determination of protein concentration via Raman measurements. Exploiting the CGSS, we successfully measured the SERS spectra of Alzheimer's biomarkers, tau protein and amyloid β, based on which secondary structural changes were analyzed quantitatively.

Fabrication of single-crystalline gold nanowires on cellulose nanofibers

Cellulose nanofibers (CNF) are promising nanomaterials for functional inks and printed sensors, although the potential applications are currently limited by the available functionalization methods. This work outlines a convenient method to grow a novel and highly conductive network of single-crystalline gold nanowires (AuNW) on CNF for use in conductive inks and printed sensors. The CNF are able to reduce Au (III) precursors to Au (0) monomers and generate nucleation sites for the subsequent monomer-by-monomer growth of Au nanocrystals; sodium citrate is used to control the reduction kinetics and the crystal growth. The growth of these AuNW/CNF materials is a three-step process of redox reaction, isotropic nucleation and anisotropic crystallization: the morphology and crystal structure of Au nanocrystals on CNF can be controlled by adjusting the reaction temperature and concentrations of citrate and CNF. The AuNW/CNF materials obtained have been formulated into highly conductive and atmospherically stable inks for use in either directly writing or screen printing. We have demonstrated AuNW/CNF-printed sensors with highly controllable electrical conductivity as well as excellent stability against rinsing and immersion by water and ethanol.

A Strategy toward Realizing Ultrashort Channels and Microstructures Array by Piezoelectric Inkjet Printing

Inkjet printing has been proved to be a powerful tool in the cost-effective ambient deposition of functional materials for the fabrication of electronic devices in the past decades. However, restricted by equipment and inks, the feature size of printed dots or lines with conventional inkjet printing is usually limited to several tens of micrometers, which could not fit the requirements for the fabrication of large-area, high-resolution microscale, even nanoscale, structures. Therefore, various technical means were developed for breaking the equipment limits. Here, we report a strategy for realizing ultrashort channels and homogeneous microstructures arrays by a conventional piezoelectric inkjet printing technique without any additional pre-mask process on the substrate. This strategy extends application of piezoelectric inkjet printing technique to biological and technological areas.

Tunable Coffee Ring Formation on Polycarbonate Nanofiber Film for Sensitive SERS Detection of Phenylalanine in Urine

A new method based on the coffee ring effect was developed for improving the sensitivity, simplicity, and robustness of surface-enhanced Raman scattering (SERS) in determining trace levels of analytes. In this method, a polyvinylpyrrolidone (PVP)-stabilized silver colloidal (AgC) solution was first prepared and mixed with a sample solution. Following deposition of the mixture solution on a solid substrate with a rough surface, a coffee ring was formed once the solvent had evaporated. The formation of a coffee ring not only concentrated the analyte but also reduced the space between silver nanoparticles (AgNPs) to strengthen the hotspot effect, thereby considerably improving SERS sensitivity. To strengthen the coffee ring effect further, the surface roughness of the solid support and PVP content of the AgC solution were investigated. The results indicated that an increase in surface roughness reduced the size of the coffee rings, whereas the addition of PVP not only stabilized the AgNPs but also improved the compactness of the coffee rings. When applying the proposed method to determine the phenylalanine (Phe) level in urine for rapid screening of the phenylketonuria disorder, strong chemical interference from uric acid (UA), which is a major component in urine, was observed. To minimize the interference from UA, ZnO powder was applied to the urine sample to adsorb UA prior to SERS detection. After cleaning by using ZnO, the SERS signals of Phe were revealed for quantitative purposes. Under the optimized conditions, both the sensitivity and reproducibility of SERS measurement considerably improved. Quantitative analyses revealed that the developed method is highly feasible for the rapid determination of Phe in real samples.

Desiccation Patterns of Plasma Sessile Drops

The renewed interest in plasma desiccation patterns focuses on the potential of these patterns to be developed into a platform of low-cost and facile diagnostic methods to interpret health conditions of donors. During desiccation, several physical mechanisms are simultaneously acting on the plasma sessile drop; these include material redistribution, buildup/release of local internal stresses, protein aggregation, and salt crystallization. After desiccation, cracking patterns and "superimposed" crystal-like patterns are formed. It has been reported that these characteristic patterns were influenced by changes in plasma compositions caused by diseases. Potential applications of these patterns in diagnosis are, however, limited by our understanding of formation mechanisms of cracking patterns and chemical compositions of crystal-like patterns. To address these limitations, this research studied morphologies of desiccated plasma patterns and the influence of sodium chloride to the pattern formation at both macroscopic and microscopic levels. Experimental results show that cracking patterns of plasma from healthy adults form throughout the desiccated deposit; propagation directions of cracks are found to have correlations to local dominant stresses, which are governed by the development of gelation. Crystal-like patterns are located in the drop center, which are caused by the heterogeneous distribution of macromolecular proteins and sodium chloride within the plasma sessile drop during desiccation; these patterns are influenced by the concentration of sodium chloride. With the increase of the concentration of sodium chloride, the distribution area of crystal-like patterns enlarges; whereas, the number of cracks decreases.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

Visualized Quantitation of Trace Nucleic Acids Based on the Coffee-Ring Effect on Colloid-Crystal Substrates

We report a visualized quantitative detection method for nucleic acid amplification tests based on the coffee-ring effect on colloid-crystal substrates. The solution for loop-mediated isothermal amplification (LAMP) of DNA is drop cast on a colloid-crystal surface. After complete drying, a coffee ring containing the LAMP byproduct (i.e., magnesium pyrophosphate) is formed, and it is found that the width of the coffee ring is linearly correlated to the logarithm of the original DNA concentration before the isothermal amplification. Importantly, compared with other substrates, we found that the colloid-crystal substrate is an appropriate substrate for carrying out the assay of high sensitivity. On the basis of these findings, we develop a coffee-ring-based assay for quantitative readout of trace DNA in a sample. The assay requires 0.50 μL of the sample and is completed in 5 min in a homemade chamber with constant humidity. Semiquantitative detection of trace DNA is performed using naked eyes. With the use of a smartphone, the DNA in a sample can be quantitatively detected with a limit of detection of 20 copies.

Controlling the contact angle of biological sessile drops for study of their desiccated cracking patterns

Current exploration of cracking patterns of desiccated biological sessile drops as a new approach of scientific research is progressing rapidly. It has been proposed that biological fluids are naturally capable of storing information. Cracking patterns of desiccated biological sessile drops have the potential to provide a facile means to study the links between compositions of biofluids, their structures and their functions. This potential is, however, limited by our current inability to control the influences of non-pathological factors on cracking patterns. Among the non-pathological factors, the initial sessile drop contact angle has a strong influence on cracking patterns through affecting the material transport and stress distributions within the drop. In this work, we developed a method to control the initial drop contact angle on a glass surface to enable the investigation of the contact angle-induced pattern changes in a biological sessile drop. Human blood was selected as the biofluid in this study, because of its richness in cracking patterns. It has been found that the increase in the initial contact angle enlarges the orthoradial cracks close to the drop edge and compresses the width of the peripheral region. We have also concluded that the number of cracks in the central region of the desiccated pattern can be correlated with the drop contact angle. This work also provides a novel protocol for fabricating standardized substrates for studies of desiccation patterns of biological and other complex colloidal fluids.

Large-Scale Fabrication of Ultrasensitive and Uniform Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Pesticides

Technology transfer from laboratory into practical application needs to meet the demands of economic viability and operational simplicity. This paper reports a simple and convenient strategy to fabricate large-scale and ultrasensitive surface-enhanced Raman scattering (SERS) substrates. In this strategy, no toxic chemicals or sophisticated instruments are required to fabricate the SERS substrates. On one hand, Ag nanoparticles (NPs) with relatively uniform size were synthesized using the modified Tollens method, which employs an ultra-low concentration of Ag⁺ and excessive amounts of glucose as a reducing agent. On the other hand, when a drop of the colloidal Ag NPs dries on a horizontal solid surface, the droplet becomes ropy, turns into a layered structure under gravity, and hardens. During evaporation, capillary flow was burdened by viscidity resistance from the ropy glucose solution. Thus, the coffee-ring effect is eliminated, leading to a uniform deposition of Ag NPs. With this method, flat Ag NPs-based SERS active films were formed in array-well plates defined by hole-shaped polydimethylsiloxane (PDMS) structures bonded on glass substrates, which were made for convenient detection. The strong SERS activity of these substrates allowed us to reach detection limits down to 10⁻¹⁴ M of Rhodamine 6 G and 10⁻¹⁰ M of thiram (pesticide).

Understanding desiccation patterns of blood sessile drops

Desiccation of a blood sessile drop on a glass surface leads to the formation of interesting cracking patterns. These desiccation patterns have been identified to have three characteristic regions, i.e., peripheral, coronal and central regions. Driving forces responsible for the formation of cracking patterns are the redistribution of colloidal materials driven by a "coffee ring" effect and the time- and location-dependent development of internal stresses caused by water evaporation and progressive gelation from the drop edge to its center. Since the concentrations of colloidal materials, i.e., cellular components, protein macromolecules and other constituents (glucose, bilirubin and lipids) in blood, influence the cracking patterns, an understanding of these patterns can potentially reveal clues for the evaluation of health conditions and offer a low-cost diagnostic tool for human diseases. This study presents a mechanistic analysis of the pattern formation in desiccating blood sessile drops. We focus on the build-up and release of internal stresses by examining the cracking process. Optical and scanning electron microscopes (SEM) were used to capture the initiation, propagation and directions of cracks in different regions. The propagation and widening of orthoradial and radial cracks in relation to the adhesion and cohesion of the blood sessile drops were observed and characterized. New microscopic insights into internal stress releasing processes provide a new understanding of physical events occurring underneath the gelled film of the blood sessile drop and differences in the distribution of strain energy in different regions, which will aid our understanding of different cracking patterns in those regions.