Oppositely-charged silver nanoparticles enable selective SERS molecular enhancement through electrostatic interactions

Label-free surface-enhanced Raman spectroscopy (SERS) has attracted extensive attention as an emerging technique for molecular phenotyping of biological samples. However, the selective enhancement property of SERS mediated by complicated interactions between substrates and analytes is unfavorable for molecular profiling. The electrostatic force is among the most dominating interactions that can cause selective adsorption of molecules to charged substrates. This means if only negatively- or positively-charged SERS substrates are applied, then considerable SERS information from a portion of analytes would be lost, hindering comprehensive SERS sensing. In this work, we utilize both negatively- and positively-charged colloidal silver (Ag) nanoparticles (NPs) to detect various charged molecules. The negatively-charged citrate-stabilized Ag and the positively-charged Ag prepared via a cetyltrimethyl-ammonium chloride-based charge reversal protocol have been adopted as SERS substrates. The Ag NPs are all relatively well-dispersed with good uniformity. After applying the oppositely-charged NPs to the detection of charged molecules, we find the SERS results explicitly demonstrate the electrostatically-driven SERS selective enhancement, which is further supported and clarified by molecular electrostatic potential calculations. Our work highlights the importance of developing SERS substrates modified with appropriate surface charges for various analytes, and enlightens us that potentially more molecular SERS information can be acquired from complex bio-samples using combinations of oppositely-charged substrates.

Unveiling Electrostatic Interaction Modulated Sensitive and Selective SERS Detection for Negatively Charged Molecules

Strengthening the interaction between the target and SERS substrate is crucial for sensitive SERS detection; we thereby explored the molecular structure-dependent SERS sensitivity for negatively charged targets on the positively charged SERS substrate. Both experimental and theoretical studies confirm that the SERS sensitivity is determined by the electrostatic interaction between the target and linker. This interaction is not only manipulated by the protonation capacity of the linker and its surface adsorption configuration and geometry but also significantly determined by the target's structure, encompassing electronegativity and the number of interaction sites. The optimized interaction leads to a marked improvement in detection sensitivity of up to 1-3 orders of magnitude. The interaction mechanism revealed in this work not only provides theoretical guidance and technical support for electrostatically driven SERS detection but also offers a conceptual framework that can be extended to various SERS detections based on diverse surface forces.

Plasmonic surface-enhanced Raman scattering nano-substrates for detection of anionic environmental contaminants: Current progress and future perspectives

Surface-enhanced Raman scattering spectroscopy (SERS) is a powerful technique of vibrational spectroscopy based on the inelastic scattering of incident photons by molecular species. It has unique properties such as ultra-sensitivity, selectivity, non-destructivity, speed, and fingerprinting properties for analytical and sensing applications. This enables SERS to be widely used in real-world sample analysis and basic plasmonic mechanistic studies. However, the desirable properties of SERS are compromised by the high cost and low reproducibility of the signals. The development of multifunctional, stable and reusable nano-engineered SERS substrates is a viable solution to circumvent these drawbacks. Recently, plasmonic SERS active nano-substrates with various morphologies have attracted the attention of researchers due to promising properties such as the formation of dense hot spots, additional stability, tunable and controlled morphology, and surface functionalization. This comprehensive review focused on the current advances in the field of SERS active nanosubstrates suitable for the detection and quantification of anionic environmental pollutants. The common fabrication methods, including the techniques for morphological adjustments and surface modification, substrate categories, and the design of nanotechnologically fabricated plasmonic SERS substrates for anion detection are systematically presented. Here, the need for the design, synthesis, and functionalization of SERS nano-substrates for anions of great environmental importance is explained in detail. In addition, the broad categories of SERS nano-substrates, namely colloid-based SERS substrates and solid-support SERS substrates are discussed. Moreover, a brief discussion of SERS detection of certain anionic pollutants in the environment is presented. Finally, the prospects in the fabrication and commercialization of pilot-scale handheld SERS sensors and the construction of smart nanosubstrates integrated with novel amplifying materials for the detection of anions of environmental and health concern are proposed.

Deep purification of copper from Cu(II)-EDTA acidic wastewater by Fe(III) replacement/diethyldithiocarbamate precipitation

Cu(II)-EDTA is a highly stable typical metal-organic complex in a wide pH range (3.0-12.0) and it is difficult to deeply purify Cu(II) by conventional precipitation methods. In this study, Fe(III) replacement/diethyldithiocarbamate (DDTC) precipitation combined process is proposed as a promising strategy to achieve the deep purification of Cu(II) from Cu(II)-EDTA acidic wastewater. The underlying mechanism has also been systematically elucidated by chemical equilibriums, experiments, and density functional theory (DFT) calculations, laying a foundation for the development and application. Chemical equilibriums show that Fe(III) replacement favors the stoichiometric release of Cu(II) from Cu(II)-EDTA and the formation of Fe(III)-EDTA complex under acidic conditions. Experimentally, Cu(II) is removed (over 99.99%) and deeply purified (under 0.008 mg/L) under the optimal conditions, which is lower than the most stringent discharge standards of copper ions in electroplating effluent (<0.5 mg/L, China). DFT calculations reveal that DDTC could further precipitate the released free copper ions via the carbon disulfide (-C(=S)-S) chelating group while exhibiting a slight effect on the Fe(III) in Fe(III)-EDTA. Considering these results, the electronic structures of Cu(II) and Fe(III), as well as their interaction with EDTA and DDTC ligands, are discussed to understand the mechanism of Fe(III)/DDTC process. By introducing a low dosage of Fe(III), the DDTC could efficiently purify Cu(II) from the Cu(II)-EDTA acid wastewater and realize the near-zero discharge of metal pollutants in metal-organic complex wastewater. It is believed that the main findings may benefit the water pollution reduction and comprehensive recycling of metal resources.

Positively-charged plasmonic nanostructures for SERS sensing applications

Surface-enhanced Raman (SERS) spectroscopy has been establishing itself as an ultrasensitive analytical technique with a cross-disciplinary range of applications, which scientific growth is triggered by the continuous improvement in the design of advanced plasmonic materials with enhanced multifunctional abilities and tailorable surface chemistry. In this regard, conventional synthetic procedures yield negatively-charged plasmonic materials which can hamper the adhesion of negatively-charged species. To tackle this issue, metallic surfaces have been modified via diverse procedures with a broad array of surface ligands to impart positive charges. Cationic amines have been preferred because of their ability to retain a positive zeta potential even at alkaline pH as well as due to their wide accessibility in terms of structural features and cost. In this review, we will describe and discuss the different approaches for generating positively-charged plasmonic platforms and their applications in SERS sensing.

Inorganic oxidizer detection from propellants, pyrotechnics, and homemade explosive powders using gradient elution moving boundary electrophoresis

Advancement in rapid targeted chemical analysis of homemade and improvised explosive devices is critical for the identification of explosives-based hazards and threats. Gradient elution moving boundary electrophoresis (GEMBE), a robust electrokinetic separation technique, was employed for the separation and detection of common inorganic oxidizers from frequently encountered fuel-oxidizer mixtures. The GEMBE system incorporated sample and run buffer reservoirs, a short capillary (5 cm), an applied electric field, and a pressure-driven counterflow. GEMBE provided a separation format that allowed for continuous injection of sample, selectivity of analytes, and no sample cleanup or filtration prior to analysis. Nitrate, chlorate, and perchlorate oxidizers were successfully detected from low explosive propellants (e.g., black powders and black powder substitutes), pyrotechnics (e.g., flash powder), and tertiary explosive mixtures (e.g., ammonium nitrate- and potassium chlorate-based fuel-oxidizer mixtures). Separation of these mixtures exhibited detection without interference from a plethora of additional organic and inorganic fuels, enabled single particle analysis, and demonstrated semiquantitative capabilities. The bulk counterflow successfully excluded difficult components from fouling the capillary, yielding estimated limits of detection down to approximately 10 μmol/L. Finally, nitrate was separated and detected from postblast debris collected and directly analyzed from two nitrate-based charges.

Trace Detection and Chemical Analysis of Homemade Fuel-Oxidizer Mixture Explosives: Emerging Challenges and Perspectives

The chemical analysis of homemade explosives (HMEs) and improvised explosive devices (IEDs) remains challenging for fieldable analytical instrumentation and sensors. Complex explosive fuel-oxidizer mixtures, black and smokeless powders, flash powders, and pyrotechnics often include an array of potential organic and inorganic components that present unique interference and matrix effect difficulties. The widely varying physicochemical properties of these components as well as external environmental interferents and background challenge many sampling and sensing modalities. This review provides perspective on these emerging challenges, critically discusses developments in sampling, sensors, and instrumentation, and showcases advancements for the trace detection of inorganic-based explosives.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Surface enhanced Raman scattering substrate for the detection of explosives: Construction strategy and dimensional effect

Surface-enhanced Raman spectroscopy (SERS) technology has been reported to be able to quickly and non-destructively identify target analytes. SERS substrate with high sensitivity and selectivity gave SERS technology a broad application prospect. This contribution aims to provide a detailed and systematic review of the current state of research on SERS-based explosive sensors, with particular attention to current research advances. This review mainly focuses on the strategies for improving SERS performance and the SERS substrates with different dimensions including zero-dimensional (0D) nanocolloids, one-dimensional (1D) nanowires and nanorods, two-dimensional (2D) arrays, and three-dimensional (3D) networks. The effects of elemental composition, the shape and size of metal nanoparticles, hot-spot structure and surface modification on the performance of explosive detection are also reviewed. In addition, the future development tendency and application of SERS-based explosive sensors are prospected.

Achieving Controllable MoS2 Nanostructures with Increased Interlayer Spacing for Efficient Removal of Pb(II) from Aquatic Systems

The development of new synthesis approaches for MoS₂ is necessary to achieve controlled morphologies and unique physicochemical properties that can improve its efficiency in particular applications. Herein, a facile one-step hydrothermal route is proposed to prepare controllable MoS₂ micro/nanostructures with an increased interlayer using sodium diethyldithiocarbamate trihydrate as the new S source at different pH values. To investigate the morphology, chemical composition, and structure of the MoS₂ micro/nanostructures, various characterization techniques were used. The obtained microrods, microspheres, and microrods with hairlike structures (denoted as MoS₂-N-H) were composed of MoS₂ nanosheets with increased interlayer spacing (∼1.0 nm) and utilized for the removal of Pb(II) from aquatic systems. Among the structures, MoS₂-N-H demonstrated the highest adsorption capacity (303.04 mg/g) for Pb(II) due to the existence of -S/-C/-N/-O-comprised functional groups on its surface, which led to strong Pb-S complexation and electrostatic attractions. The uptake of Pb(II) onto MoS₂-N-H followed pseudo-second-order kinetics and Freundlich isotherm. To evaluate its practical applicability, the adsorbent was employed in real mine water analysis; it was found that MoS₂-N-H could adsorb almost 100% of the Pb(II) ions in the presence of various coexisting ions. Additionally, after Pb(II) adsorption, MoS₂-N-H was transformed into PbMoO_{4- x}S _x spindlelike nanostructures, which were further used for photodegradation of an antibiotic, viz., ciprofloxacin (CIP), to avoid secondary environment waste. Thus, this investigation provides an effective one-pot approach to fabricate controllable MoS₂ micro/nanostructures with increased interlayer spacing for water treatment. The utility of these nanostructures in related supercapacitor/battery applications may also be envisaged because of their unique structural properties.

Forensic Analysis and Differentiation of Black Powder and Black Powder Substitute Chemical Signatures by Infrared Thermal Desorption-DART-MS

The trace detection and forensic analysis of black powders and black powder substitutes, directly from wipe-based sample collections, was demonstrated using infrared thermal desorption (IRTD) coupled with direct analysis in real time mass spectrometry (DART-MS). Discrete 15 s heating ramps were generated, creating a thermal desorption profile that desorbed more volatile species (e.g., organic and semivolatile inorganic compounds) at lower temperatures (250-400 °C) and nonvolatile inorganic oxidizers at high temperatures (450-550 °C). Common inorganic components of black powders (e.g., sulfur and potassium nitrate) as well as the alternative and additional organic and inorganic components of common black powder substitutes (e.g., dicyandiamide, ascorbic acid, sodium benzoate, guanidine nitrate, and potassium perchlorate) were detected from polytetrafluoroethylene-coated fiberglass collection wipes with no additional sample preparation. IRTD-DART-MS enabled the direct detection of intact inorganic salt species as nitrate adducts (e.g., [KClO4+NO3]-) and larger clusters. The larger ion distributions generated by these complex mixtures were differentiated using principal component analysis (PCA) of the mass spectra generated at two points during the thermal desorption profile (low and high temperatures), as well as at high in-source collision-induced dissociation. The PCA framework generated by the analysis of the two black powders and five black powder substitutes was used to classify samples collected from a commercial firecracker containing both flash powder and black powder. The coupling of IRTD-DART-MS and multivariate statistics demonstrated the powerful utility for detection and discrimination of trace fuel-oxidizer mixtures.

Hand-Held Femtogram Detection of Hazardous Picric Acid with Hydrophobic Ag Nanopillar SERS Substrates and Mechanism of Elasto-Capillarity

Picric acid (PA) is a severe environmental and security risk due to its unstable, toxic, and explosive properties. It is also challenging to detect in trace amounts and in situ because of its highly acidic and anionic character. Here, we assess sensing of PA under nonlaboratory conditions using surface-enhanced Raman scattering (SERS) silver nanopillar substrates and hand-held Raman spectroscopy equipment. The advancing elasto-capillarity effects are explained by molecular dynamics simulations. We obtain a SERS PA detection limit on the order of 20 ppt, corresponding attomole amounts, which together with the simple analysis methodology demonstrates that the presented approach is highly competitive for ultrasensitive analysis in the field.

DART-MS analysis of inorganic explosives using high temperature thermal desorption

An ambient mass spectrometry (MS) platform coupling resistive Joule heating thermal desorption (JHTD) and direct analysis in real time (DART) was implemented for the analysis of inorganic nitrite, nitrate, chlorate, and perchlorate salts. The resistive heating component generated discrete and rapid heating ramps and elevated temperatures, up to approximately 400 °C s^-1 and 750 °C, by passing a few amperes of DC current through a nichrome wire. JHTD enhanced the utility and capabilities of traditional DART-MS for the trace detection of previously difficult to detect inorganic compounds. A partial factorial design of experiments (DOE) was implemented for the systematic evaluation of five system parameters. A base set of conditions for JHTD-DART-MS was derived from this evaluation, demonstrating sensitive detection of a range of inorganic oxidizer salts, down to single nanogram levels. DOE also identified JHTD filament current and in-source collision induced dissociation (CID) energy as inducing the greatest effect on system response. Tuning of JHTD current provided a method for controlling the relative degrees of thermal desorption and thermal decomposition. Furthermore, in-source CID provided manipulation of adduct and cluster fragmentation, optimizing the detection of molecular anion species. Finally, the differential thermal desorption nature of the JHTD-DART platform demonstrated efficient desorption and detection of organic and inorganic explosive mixtures, with each desorbing at its respective optimal temperature.