Silver-Doped Sol−Gel Film as a Surface-Enhanced Raman Scattering Substrate for Detection of Uranyl and Neptunyl Ions

SERS detection of uranyl based on MOF-coated gold nanooctahedron hybrid

The ability to sensitively and quantitatively detect uranyl in complex samples plays a vital role in environmental monitoring. In this work, an MOF-coated gold (Au) nanohybrid was synthesized for uranyl detection by surface-enhanced Raman scattering (SERS) technology. The MOF shell not only prevents the Au nanoparticles from rapid aggregation, but also effectively enhances the Raman signal of uranyl. A detection limit of as low as 0.5 μM could be achieved in solution, which could be comparable to the previously reported ones from SERS-based approaches. Moreover, the prepared SERS-active substrate was also applied to uranyl detection in real samples.

Speciation Analysis of Metals and Metalloids by Surface Enhanced Raman Spectroscopy

The presence of metalloids and heavy metals in the environment is of critical concern due to their toxicological impacts. However, not all metallic species have the same risk level. Specifically, the physical, chemical, and isotopic speciation of the metal(loids) dictate their metabolism, toxicity, and environmental fate. As such, speciation analysis is critical for environmental monitoring and risk assessment. In the past two decades, surface-enhanced Raman spectroscopy (SERS) has seen significant developments regarding trace metal(loid) sensing due to its ultrahigh sensitivity, readiness for in situ real-time applications, and cost-effectiveness. However, the speciation of metal(loid)s has not been accounted for in the design and application of SERS sensors. In this Perspective, we examine the potential of SERS for metal(loid) speciation analysis and highlight the advantages, progress, opportunities, and challenges of this application.

Highly sensitive and selective determination of uranyl ions based on Ag/Ag2O-COF composite SERS substrate

Uranium is an essential nuclear material in civilian and military areas; however, its extensive application raises concerns about the potential safety issues in the fields of environmental protection and nuclear industry. In this study, we developed an Ag/Ag2O-COF (covalent-organic framework) composite SERS substrate to detect uranyl ions (UO22+) in environmental aqueous solutions. Herein, the strong SERS effect of uranyl adsorbed in Ag/Ag2O composite and the high adsorption efficiency of COF TpPa-1 were combined to realize the trace detection of uranyl ions. This method displayed a linear range of 10-8 mol L-1 to 10-6 mol L-1 with the detection limit of 8.9 × 10-10 mol L-1 for uranyl ions. Furthermore, common metal cations and oxo-ions hardly affected the SERS detection of uranyl, which is helpful for the trace analysis of uranyl in natural water samples. Although the proposed strategy is deployed for uranyl detection, the reusable and high-efficiency system may be expanded to trace detection of other substance with Raman activity.

Photoreduced Ag+/sodium alginate supramolecular hydrogel as a sensitive SERS membrane substrate for rapid detection of uranyl ions

BACKGROUND

In the fields of environmental monitoring and nuclear emergency, in order to obtain the relevant information of uranyl-induced environmental pollution and nuclear accident, it is necessary to establish a rapid quantitative analytical technique for uranyl ions. As a new promising technique, surface-enhanced Raman scattering (SERS) is hopeful to achieve this goal. However, uranyl ions are easily desorbed from SERS substrates under acidic conditions, and the structures of SERS substrates will be destroyed in the strong acidic aqueous solutions. Besides, the quantitative detection ability of SERS for uranyl ions needs to be promoted. Hence, it is necessary to develop new SERS substrates for accurate quantitative detection of trace uranyl in environmental water samples, especially in acidic solutions.

RESULTS

In this work, we prepared silver ions/sodium alginate supramolecular hydrogel membrane (Ag+/SA SMH membrane), and the Ag+ ions from the membrane were transformed into Ag/Ag2O complex nanoparticles under laser irradiation. The Raman signal of uranyl was strongly enhanced under the synergistic interaction of electromagnetic enhancement derived from the Ag nanoparticles and charge transfer enhancement between uranyl and Ag2O. Utilizing the peak of SA (550 cm-1) as an internal standard, a quantitative detection with a LOD of 6.7 × 10-9 mol L-1 was achieved due to a good linear relation of uranyl concentrations from 1.0 × 10-8 mol L-1 to 2 × 10-6 mol L-1. Furthermore, foreign metal ions hardly affected the SERS detection of uranyl, and the substrate could determine trace uranyl in natural water samples. Particularly, the acidity had no obvious effect on SERS signals of uranyl ions. Therefore, in addition to the detection of uranyl ions in natural water samples, the proposed strategy could also detect uranyl ions in strong acidic solutions.

SIGNIFICANCE AND NOVELTY

A simple one-step method was used to prepare an Ag+/SA SMH membrane for rapid quantitative detection of uranyl ions for the first time. The proposed substrate successfully detected uranyl ions under acidic conditions by immobilizing uranyl ion in hydrogel structure. In comparison with the previous studies, a more accurate quantitative analysis for uranyl ions was achieved by using an internal standard, and the proposed strategy could determine trace uranyl in either natural water samples or strong acidic solutions.

Advances in the Development of Innovative Sensor Platforms for Field Analysis

Sustainable growth, environmental preservation, and improvement of life quality are strategic fields of worldwide interest and cornerstones of international policies. Humanity health and prosperity are closely related to our present choices on sustainable development. The main sources of pollution concern industry, including mining, chemical companies, and refineries, wastewater treatment; and consumers themselves. In order to guide and evaluate the effects of environmental policies, diffuse monitoring campaigns and detailed (big) data analyses are needed. In this respect, the development and availability of innovative sensor platforms for field analysis and remote sensing are of crucial relevance. In this review, we provide an overview of the area, analyzing the major needs, available technologies, novel approaches, and perspectives. Among environmental pollutants that threaten the biosphere, we focus on inorganic and organic contaminants, which affect air and water quality. We describe the technologies for their assessment in the environment and then draw some conclusions and mention future perspectives opened by the integration of sensing technologies with robotics and the Internet of Things. Without the ambition to be exhaustive in such a rapidly growing field, this review is intended as a support for researchers and stakeholders looking for current, state-of-the-art, and key enabling technologies for environmental monitoring.

In Situ Surface-Enhanced Raman Spectroscopy Detection of Uranyl Ions with Silver Nanorod-Decorated Tape

Surface-enhanced Raman spectroscopy (SERS) has been utilized for rapid analysis of uranyl ions (UO2 2+) on account of its fast response and high sensitivity. However, the difficulty of fabricating a suitable SERS substrate for in situ analysis of uranyl ions severely restricts its practical application. Hence, we proposed flexible and adhesive SERS tape decorated with silver nanorod (AgNR) arrays for in situ detection of UO2 2+. The SERS tape was fabricated through a simple "paste & peel off" procedure by transferring the slanted AgNR arrays from silicon to the transparent tape surface. UO2 2+ can be easily in situ detected by placing the AgNR SERS tape into an aqueous solution or pasting it onto the solid matrix surface due to the excellent transparent feature of the tape. The proposed SERS tape with well-distributed AgNRs effectively improved the reproducibility and sensitivity for UO2 2+ analysis. UO2 2+ with concentration as low as 100 nM was easily detected. Besides, UO2 2+ adsorbed on an iron disc and rock surface also can be rapidly in situ detected. With its simplicity and convenience, the AgNR SERS tape-based SERS technique offers a promising approach for environmental monitoring and nuclear accident emergency detection.

The contribution of photoinduced charge-transfer enhancement to the SERS of uranyl(VI) in a uranyl-Ag2O complex

Charge-transfer (CT) is an important enhancement mechanism in the field of surface-enhanced Raman scattering (SERS) that typically increases the Raman intensity of molecules by as much as 10-100 times. Herein, a low-cost Ag₂O aggregates substrate was prepared via a facile chemical precipitation method, and the calculated CT-based enhancement factor of the uranyl ions adsorbed on it reached as high as 10⁵, a metal-comparable value. The efficient photoinduced CT process from the valence band of Ag₂O to the LUMO of uranyl ions under appropriate excitation sources resulted in the repulsion of the axial oxygen atoms of the OUO bond, which enhanced its polarizability, creating a more intense Raman mode. To the best of our knowledge, this study firstly reports such a strong photoinduced CT enhancement of uranyl ions, with concentrations of 10^-8 mol L^-1 or lower being detected using this Ag₂O substrate. Most importantly, this research has shown that the photoinduced CT enhancement also contributes to the SERS of uranyl ions on pure Ag substrates which have often been ascribed to the electromagnetic enhancement in previous studies. In addition, Ag₂O can be used to selectively detect uranyl ions without interference from many other molecules or ions because of the energy matching rule of the photoinduced CT process, which was readily available for uranyl detection in the environmental aqueous solution.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Rapid and sensitive detection of uranyl ion with citrate-stabilized silver nanoparticles by the surface-enhanced Raman scattering technique

Uranium contamination poses a huge threat to human health due to its widespread use in the nuclear industry and weapons. We proposed a simple and convenient wet-state SERS method for uranyl detection based on the citrate-stabilized silver nanoparticles. The effect of citrate on the detection performance was also discussed. By using the citrate as an internal reference to normalize the peak of uranyl, a quantitative analysis was achieved and a good linear relationship of uranyl concentration from 0.2 to 5 µM with the limit of detection of 60 nM was obtained. With its simplicity, convenience and cost-effectiveness, this method has great potential for the detection of other molecules also.

Utilizing Molecular Hyperpolarizability for Trace Analysis: A Surface-Enhanced Hyper-Raman Scattering Study of Uranyl Ion

Surface-enhanced hyper-Raman scattering (SEHRS), the nonlinear analog of surface-enhanced Raman scattering (SERS), provides unique spectral signatures arising from the molecular hyperpolarizability. In this work, we explore the differences between SERS and SEHRS spectra obtained from surface-bound uranyl ion. Exploiting the distinctive SEHRS bands for trace detection of the uranyl ion, we obtain excellent sensitivity (limit of detection = 90 ppb) despite the extreme weakness of the hyper-Raman effect. We observe that binding the uranyl ion to the carboxylate group of 4-mercaptobenzoic acid (4-MBA) leads to significant changes in the SEHRS spectrum, whereas the surface-enhanced Raman scattering (SERS) spectrum of the same complex is little changed. The SERS and SEHRS spectra are also examined as a function of both substituent position, using 2-MBA, 3-MBA, and 4-MBA, and the carbon chain length, using 4-mercaptophenylacetic acid and 4-mercaptophenylpropionic acid. These results illustrate that the unique features of SEHRS can yield more information than SERS in certain cases and represent the first application of SEHRS for trace analysis of nonresonant molecules.

SERS Sensors: Recent Developments and a Generalized Classification Scheme Based on the Signal Origin

Owing to its extreme sensitivity and easy execution, surface-enhanced Raman spectroscopy (SERS) now finds application for a wide variety of problems requiring sensitive and targeted analyte detection. This widespread application has prompted a proliferation of different SERS-based sensors, suggesting the need for a framework to classify existing methods and guide the development of new techniques. After a brief discussion of the general SERS modalities, we classify SERS-based sensors according the origin of the signal. Three major categories emerge from this analysis: surface-affinity strategy, SERS-tag strategy, and probe-mediated strategy. For each case, we describe the mechanism of action, give selected examples, and point out general misconceptions to aid the construction of new devices. We hope this review serves as a useful tutorial guide and helps readers to better classify and design practical and effective SERS-based sensors.

Matrix-Independent Surface-Enhanced Raman Scattering Detection of Uranyl Using Electrospun Amidoximated Polyacrylonitrile Mats and Gold Nanostars

Reproducible detection of uranyl, an important biological and environmental contaminant, from complex matrixes by surface-enhanced Raman scattering (SERS) is successfully achieved using amidoximated-polyacrylonitrile (AO-PAN) mats and carboxylated gold (Au) nanostars. SERS detection of small molecules from a sample mixture is traditionally limited by nonspecific adsorption of nontarget species to the metal nanostructures and subsequent variations in both the vibrational frequencies and intensities. Herein, this challenge is overcome using AO-PAN mats to extract uranyl from matrixes ranging in complexity including HEPES buffer, Ca(NO₃)₂ and NaHCO₃ solutions, and synthetic urine. Subsequently, Au nanostars functionalized with carboxyl-terminated alkanethiols are used to enhance the uranyl signal. The detected SERS signals scale with uranyl uptake as confirmed using liquid scintillation counting. SERS vibrational frequencies of uranyl on both hydrated and lyophilized polymer mats are largely independent of sample matrix, indicating less complexity in the uranyl species bound to the surface of the mats vs in solution. These results suggest that matrix effects, which commonly limit the use of SERS for complex sample analysis, are minimized for uranyl detection. The presented synergistic approach for isolating uranyl from complex sample matrixes and enhancing the signal using SERS is promising for real-world sample detection and eliminates the need of radioactive tracers and extensive sample pretreatment steps.

Twinned copper nanoparticles modulated with electrochemical deposition for in situ SERS monitoring

The SERS response of the Cu deposits depends on the deposition time and reaches its maximum value at about 150 s because of the formation of peanut-like copper particles.

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.

Self-assembly of silver nanoparticles as high active surface-enhanced Raman scattering substrate for rapid and trace analysis of uranyl(VI) ions

A facile surface-enhanced Raman scattering (SERS) substrate based on the self-assembly of silver nanoparticles on the modified silicon wafer was obtained, and for the first time, an advanced SERS analysis method basing on this as-prepared substrate was established for high sensitive and rapid detection of uranyl ions. Due to the weakened bond strength of OUO resulting from two kinds of adsorption of uranyl species ("strong" and "weak" adsorption) on the substrate, the ν₁ symmetric stretch vibration frequency of OUO shifted from 871cm^-1 (normal Raman) to 720cm^-1 and 826cm^-1 (SERS) along with significant Raman enhancement. Effects of the hydrolysis of uranyl ions on SERS were also investigated, and the SERS band at ~826cm^-1 was first used to approximately define the constitution of uranyl species at trace quantity level. Besides, the SERS intensity was proportional to the variable concentrations of uranyl nitrate ranging from 10^-7 to 10^-3molL^-1 with an excellent linear relation (R²=0.998), and the detection limit was ~10^-7molL^-1. Furthermore, the related SERS approach involves low-cost substrate fabrication, rapid and trace analysis simultaneously, and shows great potential applications for the field assays of uranyl ions in the nuclear fuel cycle and environmental monitoring.

Precisely Determining Ultralow level UO2(2+) in Natural Water with Plasmonic Nanowire Interstice Sensor

Uranium is an essential raw material in nuclear energy generation; however, its use raises concerns about the possibility of severe damage to human health and the natural environment. In this work, we report an ultrasensitive uranyl ion (UO₂²⁺) detection method in natural water that uses a plasmonic nanowire interstice (PNI) sensor combined with a DNAzyme-cleaved reaction. UO₂²⁺ induces the cleavage of DNAzymes into enzyme strands and released strands, which include Raman-active molecules. A PNI sensor can capture the released strands, providing strong surface-enhanced Raman scattering signal. The combination of a PNI sensor and a DNAzyme-cleaved reaction significantly improves the UO₂²⁺ detection performance, resulting in a detection limit of 1 pM and high selectivity. More importantly, the PNI sensor operates perfectly, even in UO₂²⁺-contaminated natural water samples. This suggests the potential usefulness of a PNI sensor in practical UO₂²⁺-sensing applications. We anticipate that diverse toxic metal ions can be detected by applying various ion-specific DNA-based ligands to PNI sensors.

The synthesis of controlled shape nanoplasmonic silver-silica structures by combining sol-gel technique and direct silver reduction

In this work, we have obtained nanoplasmonic silver structures deposited on the glass substrates by combining sol-gel technology and direct silver ion reduction on the film surfaces. The key point of the work was the usage of polyethylene glycol 400 (PEG 400) both as the pore former and reducing agent for silver ions. We have investigated the influence of PEG 400 amount on the formation of silver nanoparticles on the film surface. It was found that control of PEG 400 amount in the sols allows the creation of porous films with specific organized silver nanoparticles or clusters on the surface. Optical, morphological and structural characteristics of the structures were measured and studied. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used for nanostructure size and shape characterization. We were able to form a 40- to 200-nm-diameter ring-type, spherical and self-assembled nanoparticles on the film surface. The results of UV-vis absorbance spectra have shown the high quality of plasmonic structures with plasmon resonance wavelength in the region between 470 and 480 nm. The synthesized silica films decorated with silver nanoparticles were tested as substrates for the surface-enhanced Raman spectroscopy (SERS) and showed an enhancement relative to micro-Raman of more than 200 times.

Silver nanoparticle decorated reduced graphene oxide (rGO) nanosheet: a platform for SERS based low-level detection of uranyl ion

Herein, a simple wet-chemical pathway has been demonstrated for the synthesis of silver nanoparticle conjugated reduced graphene oxide nanosheets where dimethylformamide (DMF) is judiciously employed as an efficient reducing agent. Altogether, DMF reduces both silver nitrate (AgNO3) and graphene oxide (GO) in the reaction mixture. Additionally, the presence of polyvinylpyrolidone (PVP) assists the nanophasic growth and homogeneous distribution of the plasmonic nanoparticle Ag(0). Reduction of graphene oxide and the presence of aggregated Ag NPs on reduced graphene oxide (rGO) nanosheets are confirmed from various spectroscopic techniques. Finally, the composite material has been exploited as an intriguing platform for surface enhanced Raman scattering (SERS) based selective detection of uranyl (UO2(2+)) ion. The limit of detection has been achieved to be as low as 10 nM. Here the normal Raman spectral (NRS) band of uranyl acetate (UAc) at 838 cm(-1) shifts to 714 and 730 cm(-1) as SERS bands for pH 5.0 and 12.0, respectively. This distinguished Raman shift of the symmetric stretching mode for UO2(2+) ion is indicative of pronounced charge transfer (CT) effect. This CT effect even supports the higher sensitivity of the protocol toward UO2(2+) over other tested oxo-ions. It is anticipated that rGO nanosheets furnish a convenient compartment to favor the interaction between Ag NPs and UO2(2+) ion through proximity induced adsorption even at low concentration.

Sensitive cylindrical SERS substrate array for rapid microanalysis of nucleobases

In this work, a cylindrical-substrate array for surface-enhanced Raman scattering (SERS) measurements was developed to enable analysis of nucleobases in a few microliters of liquid. To eliminate uncertainties associated with SERS detection of aqueous samples, a new type of cylindrical SERS substrate was designed to confine the aqueous sample at the tip of the SERS probe. Poly(methyl methacrylate) (PMMA) optical fibers in a series of different diameters were used as the basic substrate. A solution of poly(vinylidene fluoride)/dimethylformamide (PVDF/DMF) was used to coat the tip of each fiber to increase the surface roughness and facilitate adsorption of silver nanoparticles (AgNPs) for enhancing Raman signals. A chemical reduction method was used to form AgNPs in and on the PVDF coating layer. The reagents and reaction conditions were systematically examined with the aim of estimating the optimum parameters. Unlike the spreading of aqueous sample on most SERS substrates, particularly flat ones, the new SERS substrates showed enough hydrophobicity to restrict aqueous sample to the tip area, thus enabling quantitative analysis. The required volume of sample could be as low as 1 μL with no need for a drying step in the procedure. By aligning the cylindrical SERS substrates into a solid holder, an array of cylindrical substrates was produced for mass analysis of aqueous samples. This new substrate improves both reproducibility and sensitivity for detection in aqueous samples. The enhancement factor approaches 7 orders in magnitude with a relative standard error close to 8%. Using the optimized conditions, nucleobases of adenine, cytosine, thymine, and uracil could be detected with limits approaching a few hundreds nanomolar in only a few microliters of solution.

Engineering metallic nanostructures for plasmonics and nanophotonics

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

Silver nanoparticle-treated filter paper as a highly sensitive surface-enhanced Raman scattering (SERS) substrate for detection of tyrosine in aqueous solution

Highly sensitive SERS substrates based on deposition of silver nanoparticles on commercially available filter paper were prepared in this work, and used to overcome problems found in analyses of aqueous samples. To prepare silver nanoparticle- (AgNP) doped filter substrates, a silver mirror reaction was used. The procedures for substrate preparation were systematically optimized. Pretreatment of filter paper, reaction time, temperature, and concentration of reagents for silver mirror reactions were studied. The morphologies of the resulting substrates were characterized by field-emission scanning electron microscopy (FE-SEM) and correlated with the SERS signals by probing with p-nitrothiophenol (pNTP). Filter papers with different pretreatments were found to have different sizes and distributions of AgNPs. The best performance was found when filter paper was pre-treated with ammonia solution before growth of AgNPs. Based on the SEM images, the resulting AgNPs had roughly spherical shape with a high degree of uniformity. The silver-coated filter paper substrates provide much higher SERS signals compared to glass substrates and the reproducibility was improved significantly. Based on statistical analyses, the relative standard deviations for substrate-to-substrate and spot-to-spot were both were less than 8% and the enhancement factors for the substrates were, in general, higher than 107. The SERS substrates were used to selectively detect tyrosine in aqueous solution. Results indicate that filter-based SERS substrates are highly suited to detection of tyrosine. Compared to glass-based SERS substrates, 50 times more SERS signal was observed in detection of tyrosine. The linear range can be up to 100 μM with a detection limit of 625 nM (SN(-1)=3).

Bio-imaging, detection and analysis by using nanostructures as SERS substrates

Surface-enhanced Raman scattering (SERS) is a phenomenon that occurs on nanoscale-roughed metallic surface. The magnitude of the Raman scattering signal can be greatly enhanced when the scatterer is placed in the very close vicinity of the surface, which enables this phenomenon to be a highly sensitive analytical technique. SERS inherits the general strongpoint of conventional Raman spectroscopy and overcomes the inherently small cross section problem of a Raman scattering. It is a sensitive and nondestructive spectroscopic method for biological samples, and can be exploited either for the delivery of molecular structural information or for the detection of trace levels of analytes. Therefore, SERS has long been regarded as a powerful tool in biomedical research. Metallic nanostructure plays a key role in all the biomedical applications of SERS because the enhanced Raman signal can only be obtained on the surface of a finely divided substrate. This review focuses on progress made in the use of SERS as an analytical technique in bio-imaging, analysis and detection. Recent progress in the fabrication of SERS active nanostructures is also highlighted.

Characterization and detection of uranyl ion sorption on silver surfaces using surface enhanced Raman spectroscopy

The study of the chemical behavior of uranyl species and its rapid detection is of primary environmental and nonproliferation concern. Herein, we report on a surface enhanced Raman spectroscopic study of uranyl ion (UO(2)(2+)) sorption onto the thermally vapor deposited silver particle surface. The ability of vibrational spectroscopy to characterize surface phenomenon and the remarkable sensitivity of surface enhanced Raman spectroscopy (SERS) have been introduced as an appropriate combination for the surface characterization and detection of UO(2)(2+) onto the silver surface. The appearance of symmetric stretching frequency of UO(2)(2+) around 700 cm(-1) and the disappearance of the 854 cm(-1) band are attributed to the development of a chemical bond between silver surface and uranyl species. The effects of temperature, solute-surface interaction time, and pH have been studied using silver modified polypropylene filter (PPF) substrates. Results show that under appropriate conditions, the concentration of uranyl ion as low as 20 ng/mL can be easily detected using the discussed SERS approach without any surface modification of silver nanoparticles. Moreover, an alternative SERS approach of uranyl detection is demonstrated using nanolithographically fabricated SERS substrates.

Influences of composition on electroless deposition of silver nanoparticles on glass substrates for surface-enhanced Raman scattering measurements

A silver-mirror reaction was used to prepare active substrates for surface-enhanced Raman scattering (SERS). Glass plates were surface treated with a thin layer of silver nanoparticles (Ag-NPs). The factors influencing the performance of the SERS substrates were systematically studied. Factors included concentrations, species of complexing agents, and reducing agents. p-Nitrothiophenol (pNTP) was used to test the surfaces with Ag-NPs, with the observed signals used to compare the performance resulting from different reducing and complexing agents. The morphologies of the Ag-NPs formed by different reaction conditions were also examined by scanning electron microscope (SEM) and correlated with the SERS signals. Reducing agents included formaldehyde, sodium tartrate, and several carbohydrates. The results indicate that the use of glucose as a reducing agent produced the most suitable Ag-NPs for SERS measurements. Complexing agents of ammonia and ethyl amine offered the best performances. The optimal concentration of complexing agent was found to be approximately six times the concentration of silver ions. With a reaction time of 2 min, the optimized concentrations of glucose and silver nitrate were 0.5 M and 50 mM, respectively. In general, the enhancement factor was on the order of 10(5) to 10(6) for the substrates prepared in this work.

Multiplexed microfluidic surface-enhanced Raman spectroscopy

Over the past few decades, surface-enhanced Raman spectroscopy (SERS) has garnered respect as an analytical technique with significant chemical and biological applications. SERS is important for the life sciences because it can provide trace level detection, a high level of structural information, and enhanced chemical detection. However, creating and successfully implementing a sensitive, reproducible, and robust SERS active substrate continues to be a challenging task. Herein, we report a novel method for SERS that is based upon using multiplexed microfluidics (MMFs) in a polydimethylsiloxane platform to perform parallel, high throughput, and sensitive detection/identification of single or various analytes under easily manipulated conditions. A facile passive pumping method is used to deliver Ag colloids and analytes into the channels where SERS measurements are done under nondestructive flowing conditions. With this approach, SERS signal reproducibility is found to be better than 7%. Utilizing a very high numerical aperture microscope objective with a confocal-based Raman spectrometer, high sensitivity is achieved. Moreover, the long working distance of this objective coupled with an appreciable channel depth obviates normal alignment issues expected with translational multiplexing. Rapid evaluation of the effects of anion activators and the type of colloid employed on SERS performance are used to demonstrate the efficiency and applicability of the MMF approach. SERS spectra of various pesticides were also obtained. Calibration curves of crystal violet (non-resonant enhanced) and Mitoxantrone (resonant enhanced) were generated, and the major SERS bands of these analytes were observable down to concentrations in the low nM and sub-pM ranges, respectively. While conventional random morphology colloids were used in most of these studies, unique cubic nanoparticles of silver were synthesized with different sizes and studied using visible wavelength optical extinction spectrometry, scanning electron microscopy, and the MMF-SERS approach.

Raman scattering study of GaN nanostructures obtained by bottom-up and top-down approaches

GaN nanocolumnar structures were grown by plasma-assisted molecular beam epitaxy (PAMBE) and also fabricated by electron cyclotron resonance reactive ion etching (ECR-RIE) of a compact GaN film parallel to the [111] direction of the Si(111) substrates. Scanning electron microscopy shows that the nanocolumns fabricated by PAMBE have a length of about 300-500 nm with diameters ranging from 20 to 150 nm while nanowhiskers formed by RIE have diameters of 40-80 nm and a height between 1.4 and 1.7 µm. A comparative study of the vibrational spectrum (including optical and interface phonons) of the nanostructures using conventional macro-Raman and micro-Raman scattering as well as surface-enhanced Raman scattering is presented.

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.

Platinum nanoparticles-doped sol–gel/carbon nanotubes composite electrochemical sensors and biosensors

Platinum nanoparticle-doped sol-gel solution is prepared and used as a binder for multi-walled carbon nanotubes (CNT) for the fabrication of electrochemical sensors. Amine group containing sol-gel solution is selected to utilize the affinity of -NH(2) groups toward metal nanoparticles for stabilization the nanoparticles in solution. The resulting CNT-silicate material brings new capabilities for electrochemical devices by using the synergistic action of the electrocatalytic activity of Pt nanoparticles and CNT. The combined electrocatalytic activity permits low-potential detection of hydrogen peroxide with remarkably improved sensitivity. With the incorporation of glucose oxidase within the Pt-CNT-silicate matrix, a Pt-CNT paste-based biosensor has been constructed that responds more sensitively to glucose than CNT-based biosensor. The influences of the composite of the sol-gel solution, the quantity of the solution and the Pt nanoparticles loading are examined. In pH 6.98 phosphate buffer, almost interference free determination of glucose is realized at 0.1 V versus SCE with a linear range from 1 to 25 mM, a response time <15s, and the sensitivity is 0.98 microA mM(-1)cm(-2). The sensitivity of the Pt-CNT paste-based biosensor is almost four times larger than that of the CNT-based biosensor (0.27 microA mM(-1)cm(-2) at 0.1 V). The improved electrocatalytic activity and surface renewability made the Pt-CNT-silicate system a potential platform to immobilize different enzymes for other bioelectrochemical applications.

Progress in plasmonic engineering of surface-enhanced Raman-scattering substrates toward ultra-trace analysis

This review describes advances made toward the application of surface-enhanced Raman scattering (SERS) in sensitive analysis and diagnostics. In the early sections of this review we briefly introduce the fundamentals of SERS including a discussion of SERS at the single-molecule level. Applications relevant to trace analysis, environmental monitoring, and homeland security and defense, for example high explosives and contaminant detection, are emphasized. Because the key to wider application of SERS analysis lies in the development of highly enhancing substrates, in the second half of the review we focus our attention on the extensive progress made in designing innovative soluble, supported, and ordered SERS-active nano-architectures to harness the potential of this technique toward solving current and emerging analytical tasks. No attempt or claim is made to review the field exhaustively in its entirety nor to cover all applications, but only to describe several significant milestones and progress made in these important areas and to provide some perspective on where the field is quickly moving.