Impact of nanotechnology on progress of flow methods in chemical analysis: A review

In evolution of instrumentation for analytical chemistry as crucial technological breakthroughs should be considered a common introduction of electronics with all its progress in integration, and then microprocessors which was followed by a widespread computerization. It is seems that a similar role can be attributed to the introduction of various elements of modern nanotechnology, observed with a fast progress since beginning of this century. It concerns all areas of the applications of analytical chemistry, including also progress in flow analysis, which are being developed since the middle of 20th century. Obviously, it should not be omitted the developed earlier and analytically applied planar structures like lipid membranes or self-assembled monolayers They had essential impact prior to discoveries of numerous extraordinary nanoparticles such as fullerenes, carbon nanotubes and graphene, or nanocrystalline semiconductors (quantum dots). Mostly, due to catalytic effects, significantly developed surface and the possibility of easy functionalization, their application in various stages of flow analytical procedures can significantly improve them. The application of new nanomaterials may be used for the development of new detection methods for flow analytical systems in macro-flow setups as well as in microfluidics and lateral flow immunoassay tests. It is also advantageous that quick flow conditions of measurements may be helpful in preventing unfavorable agglomeration of nanoparticles. A vast literature published already on this subject (e.g. almost 1000 papers about carbon nanotubes and flow-injection analytical systems) implies that for this reviews it was necessary to make an arbitrary selection of reported examples of this trend, focused mainly on achievements reported in the recent decade.

Spontaneous Intercluster Electron Transfer X2- + X0 → 2 X- (X = PtAu24(SCnH2n+1)18) in Solution: Promotion by Long Alkyl Chains

In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu₂₄(SC_nH_2n+1)₁₈]^2- (8e(Cn)) and [PtAu₂₄(SC_mH_2m+1)₁₈]⁰ (6e(Cm)) (n, m = 2-16), which have icosahedral Pt@Au₁₂ cores with eight and six valence electrons, respectively. The ET rate constant (k_ET) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the k_ET decreased with n + m in the range of 4-12, whereas it monotonically increased with n + m in the range of 12-32. Electrospray ionization mass spectrometry of the mixture of 8e(Cn) and 6e(Cm) detected the dimer complex 8e(Cn)·6e(Cm), the relative population of which increased with n + m. The activation energy (E_a), determined based on the Arrhenius plots for n = m, monotonically decreased with n (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e(Cn)·6e(Cm): (1) elongation of the lifetime and (2) the contraction of the distance between 8e(Cn) and 6e(Cm) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.

Large-area metal-dielectric heterostructures for surface-enhanced raman scattering

Metal-dielectric heterostructures have shown great application potentials in physics, chemistry and material science. In this work, we have designed and manufactured ordered metal-dielectric multiple heterostructures with tunable optical properties, which can be as large as the order of square centimeters in size. We experimentally realized that the surface-enhanced Raman scattering signal of the periodic multiple heterostructures increased 50 times compared with the silicon nanodisk-gold film arrays, which is attributed to the large-scale hotspots and high efficient coupling between the optical cavities and surface plasmon resonance modes. More importantly, the substrate also features a good uniformity and an excellent reproducible fabrication, which is very promising for practical applications.

Prospects of Surface-Enhanced Raman Spectroscopy for Biomarker Monitoring toward Precision Medicine

Future precision medicine will be undoubtedly sustained by the detection of validated biomarkers that enable a precise classification of patients based on their predicted disease risk, prognosis, and response to a specific treatment. Up to now, genomics, transcriptomics, and immunohistochemistry have been the main clinically amenable tools at hand for identifying key diagnostic, prognostic, and predictive biomarkers. However, other molecular strategies, including metabolomics, are still in their infancy and require the development of new biomarker detection technologies, toward routine implementation into clinical diagnosis. In this context, surface-enhanced Raman scattering (SERS) spectroscopy has been recognized as a promising technology for clinical monitoring thanks to its high sensitivity and label-free operation, which should help accelerate the discovery of biomarkers and their corresponding screening in a simpler, faster, and less-expensive manner. Many studies have demonstrated the excellent performance of SERS in biomedical applications. However, such studies have also revealed several variables that should be considered for accurate SERS monitoring, in particular, when the signal is collected from biological sources (tissues, cells or biofluids). This Perspective is aimed at piecing together the puzzle of SERS in biomarker monitoring, with a view on future challenges and implications. We address the most relevant requirements of plasmonic substrates for biomedical applications, as well as the implementation of tools from artificial intelligence or biotechnology to guide the development of highly versatile sensors.

Surface Enhanced Raman Spectroscopy: Applications in Agriculture and Food Safety

Recent global warming has resulted in shifting of weather patterns and led to intensification of natural disasters and upsurges in pests and diseases. As a result, global food systems are under pressure and need adjustments to meet the change—often by pesticides. Unfortunately, such agrochemicals are harmful for humans and the environment, and consequently need to be monitored. Traditional detection methods currently used are time consuming in terms of sample preparation, are high cost, and devices are typically not portable. Recently, Surface Enhanced Raman Scattering (SERS) has emerged as an attractive candidate for rapid, high sensitivity and high selectivity detection of contaminants relevant to the food industry and environmental monitoring. In this review, the principles of SERS as well as recent SERS substrate fabrication methods are first discussed. Following this, their development and applications for agrifood safety is reviewed, with focus on detection of dye molecules, melamine in food products, and the detection of different classes of pesticides such as organophosphate and neonicotinoids.

Interaction of the (2√3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation

We show that the adsorption site basis of the (2√3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au-S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (√3 × √3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au-S interfacial models.

Effect of the functionalisation agent on the surface-enhanced Raman scattering (SERS) spectrum: Case study of pyridine derivatives

Nowadays, the use of functionalised surface-enhanced Raman scattering (SERS) substrates has become common. These surface modifying agents notably act as Raman reporters, as sensors of biological processes (pH, redox probes) or to increase the sensitivity and/or the specificity of SERS detections. However, the effects of the functionalisation agents are deeply examined in very few studies, even though they can affect the aggregation behaviour of the SERS substrate. Moreover, depending on their concentration and on the pH, their spectral signature can be modified and they can even degrade if stored inappropriately. In this context, this paper aims at emphasising the importance of the different aspects previously listed in the selection of a functionalisation agent. Pyridine derivatives were picked out to highlight these parameters, as some of these compounds are commonly used to be grafted onto SERS substrates. Two widespread syntheses of nanoparticles were selected as SERS substrates: citrate-reduced gold and silver nanoparticles. The surface of the nanoparticles was functionalised with several pyridine derivatives at different concentrations and in several solvents. It was observed that the molecules under study had a concentration-dependent effect on nanoparticle aggregation. A stability study was furthermore conducted in order to determine the best preservation conditions of the grafting solutions. In conclusion, this paper shines a light on the relevance of the investigation of the too-often neglected behaviour of the surface modifying agents. Before their application in SERS analyses, parameters such as the label concentration should therefore be included in an experimental design to optimise the sample preparation.

SERS based monitoring of toluene vapors at ambient and elevated temperatures by using a ruffled silver nanolayer as a substrate

The authors describe a Surface enhanced Raman spectroscopy (SERS)-based method for the detection of gaseous toluene at different temperature regimes using 3D ruffled silver SERS substrates and a commercially available handheld Raman system equipped with a 785 nm laser. The 3D silver SERS substrates were synthesized via electroless deposition of silver on the ruffled sandpaper and HF-etched silicon wafers. The morphological characterization of the silver SERS substrates was carried out by atomic force microscopy and scanning electron microscopy. UV-Vis spectroscopy absorption spectra of the silver nanostructures showed plasmonic peaks at 522 nm and 731 nm. Toluene vapors were collected with a syringe at ambient temperature and at 100 °C, while SERS detection was always performed at room temperature. Toluene detection was based on the measurement of the Raman bands at 787 cm^-1 and 1003 cm^-1 (in the fingerprint region). The method allow gaseous toluene to be detected at its vapor concentrations of 522 ppm (mg/L), 261 ppm (mg/L) and 26 ppm (mg/L). Graphical abstract Schematic presentation of an original method for the detection of toluene vapors by SERS technique. The collection of toluene vapors was carried out at room and at high temperatures. The vapors were transferred to methanol by bubbling. The SERS measurements were carried out at room temperature.

Review of SERS Substrates for Chemical Sensing

The SERS effect was initially discovered in the 1970s. Early research focused on understanding the phenomenon and increasing enhancement to achieve single molecule detection. From the mid-1980s to early 1990s, research started to move away from obtaining a fundamental understanding of the phenomenon to the exploration of analytical applications. At the same time, significant developments occurred in the field of photonics that led to the advent of inexpensive, robust, compact, field-deployable Raman systems. The 1990s also saw rapid development in nanoscience. This convergence of technologies (photonics and nanoscience) has led to accelerated development of SERS substrates to detect a wide range of chemical and biological analytes. It would be a monumental task to discuss all the different kinds of SERS substrates that have been explored. Likewise, it would be impossible to discuss the use of SERS for both chemical and biological detection. Instead, a review of the most common metallic (Ag, Cu, and Au) SERS substrates for chemical detection only is discussed, as well as SERS substrates that are commercially available. Other issues with SERS for chemical detection have been selectivity, reversibility, and reusability of the substrates. How these issues have been addressed is also discussed in this review.

Plasmonic nano-protrusions: hierarchical nanostructures for single-molecule Raman spectroscopy

Classical methods for enhancing the electromagnetic field from substrates for spectroscopic applications, such as surface-enhanced Raman spectroscopy (SERS), have involved the generation of hotspots through directed self-assembly of nanoparticles or by patterning nanoscale features using expensive nanolithography techniques. A novel large-area, cost-effective soft lithographic technique involving glancing angle deposition (GLAD) of silver on polymer gratings is reported here. This method produces hierarchical nanostructures with high enhancement factors capable of analyzing single-molecule SERS. The uniform ordered and patterned nanostructures provide extraordinary field enhancements that serve as excitatory hotspots and are herein interrogated by SERS. The high spatial homogeneity of the Raman signal and signal enhancement over a large area from a self-assembled monolayer (SAM) of 2-naphthalenethiol demonstrated the uniformity of the hotspots. The enhancement was shown to have a critical dependence on the underlying nanostructure via the surface energy landscape and GLAD angles for a fixed deposition thickness, as evidenced by atomic force microscopy and scanning electron microscopy surface analysis of the substrate. The nanostructured surface leads to an extremely concentrated electromagnetic field at sharp nanoscale peaks, here referred to as 'nano-protrusions', due to the coupling of surface plasmon resonance (SPR) with localized SPR. These nano-protrusions act as hotspots which provide Raman enhancement factors as high as 10⁸ over a comparable SAM on silver. Comparison of our substrate with the commercial substrate Klarite™ shows higher signal enhancement and minimal signal variation with hotspot spatial distribution. By using the proper plasmon resonance angle corresponding to the laser source wavelength, further enhancement in signal intensity can be achieved. Single-molecule Raman spectra for rhodamine 6G are obtained from the best SERS substrate (a GLAD angle of 60°). The single-molecule spectrum is invariant over the substrate, due to the patterned ordered nanostructures (nano-protrusions).

SERS detection of arsenic in water: A review

Arsenic (As) is one of the most toxic contaminants found in the environment. Development of novel detection methods for As species in water with the potential for field use has been an urgent need in recent years. In past decades, surface-enhanced Raman scattering (SERS) has gained a reputation as one of the most sensitive spectroscopic methods for chemical and biomolecular sensing. The SERS technique has emerged as an extremely promising solution for in-situ detection of arsenic species in the field, particularly when coupled with portable/handheld Raman spectrometers. In this article, the recent advances in SERS analysis of arsenic species in water media are reviewed, and the potential of this technique for fast screening and field testing of arsenic-contaminated environmental water samples is discussed. The problems that remain in the field are also discussed and an outlook for the future is featured at the end of the article.

Hybrid nanostructures for SERS: materials development and chemical detection

This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.

Surface-Enhanced Raman Scattering: A Technique of Choice for Molecular Detection

Although surface-enhanced Raman scattering (SERS) has crossed its infancy long ago, it is yet to persuade different challenges to make it available in day-to-day applications. SERS is being criticized mainly due to the quality of the SERS analyses that uses substrates to get the giant enhancement for respective Raman signal of the target molecule. Hence, understanding the phenomena behind substrates, cost-effective development and optimization of such substrates for routine analytical purposes and utilization of modern modalities to get the insights out has become a very wide-spreading and interesting area of research. In this piece of work, several key terminologies related to SERS have been presented in brief. Since SERS is a localized surface plasmon resonance (LSPR) mediated signal-enhancing phenomena, it is indispensable to understand the correlation between LSPR excitations originated from substrate and SERS signal originated from molecules. A wide range of SERS-active substrates including scattered nanoaggregates, anisotropic assembly, two-dimensional nanostructure, multi-layered nanostructure of gold nanoparticles and colloidal approach have been used to interpret such correlation between LSPR excitations and SERS characteristics. Few exemplary applications of SERS have been also mentioned followed by typical simulative work how nanoobject behaves at different excitations and polarizations.