Controllable Self-Assembly of SERS Hotspots in Liquid Environment

Detection of organic dyes using Ag NPAs/SMP SERS substrate produced via sandpaper template-assisted lithography and liquid-liquid interface self-assembly

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive and reliable fingerprinting technique. However, its analytical capability is closely related to the quality of a SERS substrate used for the analysis. In particular, conventional colloidal substrates possess disadvantages in terms of controllability, stability, and reproducibility, which limit their application. In order to address these issues, a simple, cost-effective, and efficient SERS substrate based on silver nanoparticle arrays (Ag NPAs) and sandpaper-molded polydimethylsiloxane (SMP) was proposed in this work. Successfully prepared via template lithography and liquid-liquid interface self-assembly (LLISA), the substrate can be applied to the specific detection of organic dyes in the environment. The substrate exhibited good SERS performance, and the limit of detection (LOD) of rhodamine 6G (R6G) was shown to be 10-7 M under the optimal conditions (1000 grit sandpaper) with a relative standard deviation (RSD) of 7.76%. Moreover, the SERS signal intensity was maintained at 60% of the initial intensity after the substrate was stored for 30 days. In addition, the Ag NPAs/SMP SERS substrate was also employed to detect crystal violet (CV) and methylene blue (MB) with the LODs of 10-6 M and 10-7 M, respectively. In summary, the Ag NPAs/SMP SERS substrate prepared in this study has great potential for the detection of organic dyes in ecological environments.

Potential Regulation for Surface-Enhanced Raman Scattering Detection and Identification of Carotenoids

Surface-enhanced Raman scattering (SERS) is often impaired by the limited affinity of molecules to plasmonic substrates. Here, we use carbon fiber microelectrodes modified with silver nanoparticles as a plasmonic microsubstrate with tunable affinity for enrichment and molecular identification by SERS. The silver nanoparticles self-assemble by electrostatic interaction with diamine molecules that are electrochemically grafted onto the surface of the microelectrodes. β-carotene and trans-β-Apo-8'-carotenal, producing similar resonant SERS spectra, are employed as model molecules to study the effect of electroenrichment and SERS screening for different electrode potentials. The data show that at a characteristic electrode potential, the low affinity of polyene chains without hydrophilic groups to the substrate can be overcome. Different potentials were applied to recognize the two types of carotenoids by their typical SERS signal, and the applicability of this strategy was further confirmed in the environment of a real cell culture. The results indicate that by regulating the potential, carotenoid molecules with a similar molecular structure can be selectively quantified and identified by SERS. The developed SERS-active microelectrode is expected to help the development of portable, miniaturized point-of-care diagnostic SERS sensors.

Trends of Biosensing: Plasmonics through Miniaturization and Quantum Sensing

Despite being extremely old concepts, plasmonics and surface plasmon resonance-based biosensors have been increasingly popular in the recent two decades due to the growing interest in nanooptics and are now of relevant significance in regards to applications associated with human health. Plasmonics integration into point-of-care devices for health surveillance has enabled significant levels of sensitivity and limit of detection to be achieved and has encouraged the expansion of the fields of study and market niches devoted to the creation of quick and incredibly sensitive label-free detection. The trend reflects in wearable plasmonic sensor development as well as point-of-care applications for widespread applications, demonstrating the potential impact of the new generation of plasmonic biosensors on human well-being through the concepts of personalized medicine and global health. In this context, the aim here is to discuss the potential, limitations, and opportunities for improvement that have arisen as a result of the integration of plasmonics into microsystems and lab-on-chip over the past five years. Recent applications of plasmonic biosensors in microsystems and sensor performance are analyzed. The final analysis focuses on the integration of microfluidics and lab-on-a-chip with quantum plasmonics technology prospecting it as a promising solution for chemical and biological sensing. Here it is underlined how the research in the field of quantum plasmonic sensing for biological applications has flourished over the past decade with the aim to overcome the limits given by quantum fluctuations and noise. The significant advances in nanophotonics, plasmonics and microsystems used to create increasingly effective biosensors would continue to benefit this field if harnessed properly.

SERS Sensing Using Graphene-Covered Silver Nanoparticles and Metamaterials for the Detection of Thiram in Soil

Multilayer hyperbolic metamaterial (HMM)-based SERS substrates have received special consideration because they accommodate various propagation modes such as surface plasmonic polaritons (SPP). However, the SPP modes are difficult to generate in HMM due to their weak electric field enhancement. In this article, we designed novel SERS substrates consisting of graphene-covered AgNPs and HMM. The graphene-covered AgNPs work as an external coupling structure for hyperbolic metamaterials due to this structure exhibiting significant plasmonic effects as well as unique optical features. The localized surface plasmonic resonance (LSPR) of the graphene-covered AgNPs excited the SPP and thus formed a strong hot spot zone in the nanogap area of the graphene. The Raman experiment was performed using rhodamine 6G (R6G) and crystal violet (CV), which showed high stability and a maximum enhancement factor of 2.12 × 10⁸. The COMSOL simulation further clarified that enhanced SERS performance was due to the presence of monolayer graphene and provided an atomically flat surface for organic molecules in a more controllable manner. Interestingly, the proposed SERS structure carries out quantitative detection of thiram in soil and can satisfy the basic environmental need for pesticide residue in the soil.

Enhanced Surface Plasmon by Clusters in TiO2-Ag Composite

The surface plasmon in the composite composed of the noble metals and the semiconductors is interesting because of the various charges and the potential applications in many fields. Based on a highly ordered 2D polystyrene spheres array, the ordered composite nanocap arrays composed of TiO₂ and Ag were prepared by the co-sputtering technique, and the surface morphology was tuned by changing TiO₂ sputtering power. When TiO₂ sputtering power was 60 W and Ag sputtering power was 10 W, the composite unit arrays showed the nanocap shapes decorated by many composite clusters around. The composite clusters led to the additional local coupling of the electromagnetic fields and significant Surface-Enhanced Raman Scattering (SERS) observations, which was also confirmed by the finite-different time-domain simulation. The SERS-active substrate composed of the composite nanocaps decorated by clusters realized the accurate detection of the thiram with concentrations down to 10^-9 M.

Recent Advances in the Use of Surface-Enhanced Raman Scattering for Illicit Drug Detection

The rapid increase in illicit drug use and its adverse health effects and socio-economic consequences have reached alarming proportions in recent years. Surface-enhanced Raman scattering (SERS) has emerged as a highly sensitive analytical tool for the detection of low dosages of drugs in liquid and solid samples. In the present article, we review the state-of-the-art use of SERS for chemical analysis of illicit drugs in aqueous and complex biological samples, including saliva, urine, and blood. We also include a review of the types of SERS substrates used for this purpose, pointing out recent advancements in substrate fabrication towards quantitative and qualitative detection of illicit drugs. Finally, we conclude by providing our perspective on the field of SERS-based drug detection, including presently faced challenges. Overall, our review provides evidence of the strong potential of SERS to establish itself as both a laboratory and in situ analytical method for fast and sensitive drug detection and identification.

Controlling the Shrinkage of 3D Hot Spot Droplets as a Microreactor for Quantitative SERS Detection of Anticancer Drugs in Serum Using a Handheld Raman Spectrometer

Quantitative measurement is one of the ultimate targets for surface-enhanced Raman spectroscopy (SERS), but it suffers from difficulties in controlling the uniformity of hot spots and placing the target molecules in the hot spot space. Here, a convenient approach of three-phase equilibrium controlling the shrinkage of three-dimensional (3D) hot spot droplets has been demonstrated for the quantitative detection of the anticancer drug 5-fluorouracil (5-FU) in serum using a handheld Raman spectrometer. Droplet shrinkage, triggered by the shaking of aqueous nanoparticle (NP) colloids with immiscible oil chloroform (CHCl₃) after the addition of negative ions and acetone, not only brings the nanoparticles in close proximity but can also act as a microreactor to enhance the spatial enrichment capability of the analyte in plasmonic sites and thereby realize simultaneously controlling 3D hot spots and placing target molecules in hot spots. Moreover, the shrinking process of Ag colloid droplets has been investigated using a high-speed camera, an in situ transmission electron microscope (in situ TEM), and a dark-field microscope (DFM), demonstrating the high stability and uniformity of nanoparticles in droplets. The shrunk Ag NP droplets exhibit excellent SERS sensitivity and reproducibility for the quantitative analysis of 5-FU over a large range of 50-1000 ppb. Hence, it is promising for quantitative analysis of complex systems and long-term monitoring of bioreactions.

Liquid-liquid interfacial self-assembled triangular Ag nanoplate-based high-density and ordered SERS-active arrays for the sensitive detection of dibutyl phthalate (DBP) in edible oils

DBP, one of the phthalic acid esters (PAEs), is known as an endocrine disruptor and is toxic to humans in abnormal concentrations. Here, a high-density and ordered SERS substrate based on the self-assembly of triangular Ag nanoplate (TAgNP) arrays is developed for DBP detection. Benefiting from the ordered arrangement and sharp corners of TAgNPS, the arrays can provide sufficient and uniform hotspots for reproducible and highly active SERS effects. Using Rhodamine 6G (R6G) as a reporter molecule, the SERS enhancement factor (EF) of the TAgNP arrays was found to be as high as 1.2 × 107 and the relative standard deviation was 6.56%. As a trial for practical applications, the TAgNP array substrates were used for the detection of dibutyl phthalate (DBP) in edible oils. In this assay, edible oil samples were added to hexane as an organic phase for the formation of the TAgNP arrays, which caused DBP to be loaded at hotspots. DBP in edible oils could be identified at concentrations as low as 10-7 M. This SERS substrate based on the TAgNP arrays has great potential applications in the high sensitivity and reproducible detection of contaminants in food.