A nanosensor for TNT detection based on molecularly imprinted polymers and surface enhanced Raman scattering

A new PdMOF-loaded molecularly imprinted polyaniline nanocatalytic probe for ultratrace oxytetracycline with SERS technique

In this paper, a new Pd metal organic framework (PdMOF) surface molecularly imprinted polyaniline nanocatalytic probe (PdMOF@MIP) with dual functions of recognition and catalysis was synthesized. It is found that the PdMOF@MIP nanoprobe can not only identify OTC but also catalyze the new nanoreaction of NaH2PO2-HAuCl4 to generate gold nanoparticles (AuNPs), and the generated AuNPs could be traced by surface-enhanced Raman scattering (SERS). When OTC specifically binds to PdMOF@MIP to generate PdMOF@MIP-OTC conjugate, its catalytic effect is weakened and the analytical signal is reduced linaerly. Accordingly, a new, highly sensitive, selective and simple SERS/RRS/Abs trimode detection platform for OTC was constructed. The linear range of SERS was 0.0625 ng/mL ∼ 1.75 ng/mL and the limit of detection was 0.015 ng/mL. This new nanocatalytic probe detection strategy can also be used for the selective detection of other antibiotics such as tetracycline and doxycycline, respectively. In addition, the nanocatalytic mechanism has been investigated.

Ultrasensitive detection strategy for CAP by molecularity imprinted SERS sensor based on multiple synergistic enhancement of SiO2@AuAg with MOFs@Au signal carrier

Chloramphenicol (CAP) residue in food can cause great harm to human health, it is important to develop a rapid and sensitive method to detect CAP. Here, molecularly imprinted polymer (MIP) was combined with metal-organic frameworks@Au (MOFs@Au) collaborative construction surface-enhanced Raman spectroscopy (SERS) based aptasensor for CAP ultrasensitive detection. MOFs@Au first carried the Raman signal molecule toluidine blue (TB) and aptamer to form MOFs@Au@TB@Apt. In addition, rMIP (CAP was removed) was dropped onto the uniform three-dimensional (3D) SERS substrate SiO2@AuAg to form SiO2@AuAg@rMIP. In the presence of target CAP, it could be specifically captured with rMIP by covalent interaction and was recognised by the aptamer. During this time, SiO2@AuAg@rMIP@CAP could selectively connect MOFs@Au@TB@Apt to realise synergistic enhance the Raman signal. Based on this principle, the proposed SERS aptasensor exhibits excellent sensitivity with a detection limit of 7.59×10-13 M for CAP, providing a new strategy for trace detection in food.

Analysis of Nanostar Reshaping Kinetics for Optimal Substrate Fabrication

Gold nanostars (NS) are emerging as a versatile tool in surface-enhanced Raman scattering (SERS) applications because of their wide localized surface plasmon resonance (LSPR) tunability, simple synthesis procedure, and high SERS enhancement. These particles are commonly used in solutions with a stabilizing coating shell (e.g., thiolated molecules or silver shell). However, coatings cannot be used for the fabrication of SERS substrates as the NS have to interact with the substrate planar surface. Without coating, NS have been observed to change over time, leading to a hypochromic shift of the LSPR. To understand this shift, we synthesized surfactant-free gold NS with different spike morphologies and investigated their reshaping morphology and kinetics. Using TEM, the NS sharp spike features were observed to reshape over time. The kinetics of this process were analyzed and determined by monitoring the LSPR, which was observed to follow an exponential decay over time. We used an empirical fit for the LSPR-shift data as a function of time, which permits to predict the LSPR at a specific time based only on the initial LSPR (independently of the initial spike morphology). We show the effect of the LSPR on the SERS signal for the NS and how the SERS signal correlated to our prediction. Finally, we evaluated our approach by fabricating SERS substrates with immobilized NS and collecting the reflectance spectra. We were able to predict the substrate LSPR and aim for an optimal LSPR with an average 3% deviation. These new insights on NS reshaping can permit the fabrication of NS-based substrates with desirable optical/plasmonic properties.

A facile cost-effective electrolyte-assisted approach and comparative study towards the Greener synthesis of silica nanoparticles

Nowadays, silica nanoparticles are gaining tremendous importance because of their wide applications across different domains such as drug delivery, chromatography, biosensors, and chemosensors. The synthesis of silica nanoparticles generally requires a high percentage composition of organic solvent in an alkali medium. The eco-friendly synthesis of silica nanoparticles in bulk amounts can help save the environment and is cost-effective. Herein, efforts have been made to minimize the concentration of organic solvents used during synthesis via the addition of a low concentration of electrolytes, e.g., NaCl. The effects of electrolytes and solvent concentrations on nucleation kinetics, particle growth, and particle size were investigated. Ethanol was used as a solvent in various concentrations, ranging from 60% to 30%, and to optimize and validate the reaction conditions, isopropanol and methanol were also utilized as solvents. The concentration of aqua-soluble silica was determined using the molybdate assay to establish reaction kinetics, and this approach was also utilized to quantify the relative concentration changes in particles throughout the synthesis. The prime feature of the synthesis is the reduction in organic solvent usage by up to 50% using 68 mM NaCl. The surface zeta potential was reduced after the addition of an electrolyte, which made the condensation process faster and helped reaching the critical aggregation concentration in a shorter time. The effect of temperature was also monitored, and we obtained homogeneous and uniform nanoparticles by increasing the temperature. We found that it is possible to tune the size of the nanoparticles by changing the concentration of electrolytes and the temperature of the reaction using an eco-friendly approach. The overall cost of the synthesis can also be reduced by ∼35% by adding electrolytes.

Molecularly Imprinted Polymer-Based Sensors for Protein Detection

The accurate detection of biological substances such as proteins has always been a hot topic in scientific research. Biomimetic sensors seek to imitate sensitive and selective mechanisms of biological systems and integrate these traits into applicable sensing platforms. Molecular imprinting technology has been extensively practiced in many domains, where it can produce various molecular recognition materials with specific recognition capabilities. Molecularly imprinted polymers (MIPs), dubbed plastic antibodies, are artificial receptors with high-affinity binding sites for a particular molecule or compound. MIPs for protein recognition are expected to have high affinity via numerous interactions between polymer matrices and multiple functional groups of the target protein. This critical review briefly describes recent advances in the synthesis, characterization, and application of MIP-based sensor platforms used to detect proteins.

Protamine gold nanoclusters - based fluorescence turn-on sensor for rapid determination of Trinitrotoluene (TNT)

Determination of trace residues of 2,4,6-trinitrotoluene (TNT) is an analytical challenge as it is widely used in military, mining industry, civilian and counter-terrorism purposes. In this study, a gold nanocluster - based turn-on fluorescence sensor was developed for TNT determination. A one-pot approach was used to synthesize the fluorescent protamine - stabilized gold nanoclusters (PRT-AuNC). The proposed turn-on fluorometric sensor relies on the aggregation-induced emission enhancement mechanism. As a result of the donor-acceptor interaction between the non-fluorescent Meisenheimer anion formed from TNT and the amino groups of weakly fluorescent protamine, the PRT-AuNCs aggregate and an accompanying enhancement in fluorescence intensity is observed with a large Stokes shift (λ_ex = 300 nm, λ_em = 600 nm). The fluorescence enhancement increased linearly with TNT with an LOD of 12.44 µg/L. Similar energetic materials, common soil ions and explosive camouflage materials did not affect the proposed fluorometric sensing method. TNT in artificially contaminated soil was determined, and the results were comparable to those obtained by the HPLC-DAD system. The proposed turn-on sensor is an important tool for simple, fast, rapid and sensitive TNT determination, and has a potential to be converted to a kit format.

Bio-Inspired Imprinting Materials for Biomedical Applications

Inspired by the recognition mechanism of biological molecules, molecular imprinting techniques (MITs) are imparted with numerous merits like excellent stability, recognition specificity, adsorption properties, and easy synthesis processes, and thus broaden the avenues for convenient fabrication protocol of bio-inspired molecularly imprinted polymers (MIPs) with desirable functions to satisfy the extensive demands of biomedical applications. Herein, the recent research progress made with respect to bio-inspired imprinting materials is discussed in this review. First, the underlying mechanism and basic components of a typical molecular imprinting procedure are briefly explored. Then, emphasis is put on the introduction of diverse MITs and novel bio-inspired imprinting materials. Following these two sections, practical applications of MIPs in the field of biomedical science are focused on. Last but not least, perspectives on the remaining challenges and future development of bio-inspired imprinting materials are presented.

Template Imprinting Versus Porogen Imprinting of Small Molecules: A Review of Molecularly Imprinted Polymers in Gas Sensing

The selective sensing of gaseous target molecules is a challenge to analytical chemistry. Selectivity may be achieved in liquids by several different methods, but many of these are not suitable for gas-phase analysis. In this review, we will focus on molecular imprinting and its application in selective binding of volatile organic compounds and atmospheric pollutants in the gas phase. The vast majority of indexed publications describing molecularly imprinted polymers for gas sensors and vapour monitors have been analysed and categorised. Specific attention was then given to sensitivity, selectivity, and the challenges of imprinting these small volatile compounds. A distinction was made between porogen (solvent) imprinting and template imprinting for the discussion of different synthetic techniques, and the suitability of each to different applications. We conclude that porogen imprinting, synthesis in an excess of template, has great potential in gas capture technology and possibly in tandem with more typical template imprinting, but that the latter generally remains preferable for selective and sensitive detection of gaseous molecules. More generally, it is concluded that gas-phase applications of MIPs are an established science, capable of great selectivity and parts-per-trillion sensitivity. Improvements in the fields are likely to emerge by deviating from standards developed for MIP in liquids, but original methodologies generating exceptional results are already present in the literature.

Influence of Air Flow on Luminescence Quenching in Polymer Films towards Explosives Detection Using Drones

Explosive detection has become an increased priority in recent years for homeland security and counter-terrorism applications. Although drones may not be able to pinpoint the exact location of the landmines and explosives, the identification of the explosive vapor present in the surrounding air provides significant information and comfort to the personnel and explosives removal equipment operators. Several optical methods, such as the luminescence quenching of fluorescent polymers, have been used for explosive detection. In order to utilize sensing technique via unmanned vehicles or drones, it is very important to study how the air flow affects the luminescence quenching. We investigated the effects of air flow on the quenching efficiency of Poly(2,5-di(2′-ethylhexyl)-1,4-ethynylene) (PEE) by TNT molecules. We treated the TNT molecules incorporated into the polymer film as non-radiative recombination centers, and found that the time derivative of the non-radiative recombination rates was greater with faster air flows. Our investigations show that relatively high air flow into an optical sensing part is crucial to achieving fast PL quenching. We also found that a “continuous light excitation” condition during the exposure of TNT vapor greatly influences the PL quenching.

Development of Gold Nanoparticles Decorated Molecularly Imprinted–Based Plasmonic Sensor for the Detection of Aflatoxin M1 in Milk Samples

Aflatoxins are a group of extremely toxic and carcinogenic substances generated by the mold of the genus Aspergillus that contaminate agricultural products. When dairy cows ingest aflatoxin B1 (AFB1)−contaminated feeds, it is metabolized and transformed in the liver into a carcinogenic major form of aflatoxin M1 (AFM1), which is eliminated through the milk. The detection of AFM1 in milk is very important to be able to guarantee food safety and quality. In recent years, sensors have emerged as a quick, low–cost, and reliable platform for the detection of aflatoxins. Plasmonic sensors with molecularly imprinted polymers (MIPs) can be interesting alternatives for the determination of AFM1. In this work, we designed a molecularly–imprinted–based plasmonic sensor to directly detect lower amounts of AFM1 in raw milk samples. For this purpose, we prepared gold–nanoparticle–(AuNP)−integrated polymer nanofilm on a gold plasmonic sensor chip coated with allyl mercaptan. N−methacryloyl−l−phenylalanine (MAPA) was chosen as a functional monomer. The MIP nanofilm was prepared using the light–initiated polymerization of MAPA and ethylene glycol dimethacrylate in the presence of AFM1 as a template molecule. The developed method enabled the detection of AFM1 with a detection limit of 0.4 pg/mL and demonstrated good linearity (0.0003 ng/mL–20.0 ng/mL) under optimized experimental conditions. The AFM1 determination was performed in random dairy farmer milk samples. Using the analogous mycotoxins, it was also demonstrated that the plasmonic sensor platforms were specific to the detection of AFM1.

Targeted Mitochondrial Fluorescence Imaging-Guided Tumor Antimetabolic Therapy with the Imprinted Polymer Nanomedicine Capable of Specifically Recognizing Dihydrofolate Reductase

As we all know, inhibiting the activity of dihydrofolate reductase (DHFR) has always been an effective strategy for folate antimetabolites to treat tumors. In the past, it mainly relied on chemical drugs. Here, we propose a new strategy, (3-propanecarboxyl)triphenylphosphonium bromide (CTPB)-modified molecularly imprinted polymer nanomedicine (MIP-CTPB). MIP-CTPB prepared by imprinting the active center of DHFR can specifically bind to the active center to block the catalytic activity of DHFR, thereby inhibiting the synthesis of DNA and ultimately inhibiting the tumor growth. The modification of CTPB allows the nanomedicine to be targeted and enriched in mitochondria, where DHFR is abundant. The confocal laser imaging results show that MIP-CTPB can target mitochondria. Cytotoxicity experiments show that MIP-CTPB inhibits HeLa cell proliferation by 42.2%. In vivo experiments show that the tumor volume of the MIP-CTPB-treated group is only one-sixth of that of the untreated group. The fluorescent and paramagnetic properties of the nanomedicine enable targeted fluorescence imaging of mitochondria and T2-weighted magnetic resonance imaging of tumors. This research not only opens up a new direction for the application of molecular imprinting, but also provides a new idea for tumor antimetabolic therapy guided by targeted mitochondrial imaging.

A Simple, Rapid and Low-cost 3-Aminopropyltriethoxysilane (APTES)-based Surface Plasmon Resonance Sensor for TNT Explosive Detection

In this study, a simple, one-step organic molecule 3-aminopropyltriethoxysilane (APTES) functionalized surface plasmon resonance (SPR) sensor was developed. APTES as an organic ligand immobilized on the SPR sensor chip was used to form the Meisenheimer complex with 2,4,6-trinitrotoluene (TNT). The results of using the APTES-based SPR sensor chip show a highly selective and sensitive (ppb level: parts per billion) detection of TNT explosive. The sensor is expected to have potential for application in the fast screening of the TNT explosive.

Construction of MOF-shell porous materials and performance studies in the selective adsorption and separation of benzene pollutants

Metastable Cu2O is an attractive material for the architectural design of integrated nanomaterials. In this context, Cu2O was used as the sacrificial agent to form the core-shell structure of Cu2O@HKUST-1 by in situ growth technology. The MOFs with BOPs adsorption property were gathered together by a Cu2O etching method, and the hollow structure of the HKUST-1 shell material with fast BOP adsorption was successfully constructed. The adsorption experiments showed that the HKUST-1 shell has a good adsorption effect on nitrobenzene pollutants in wastewater. The investigation of various factors affecting the adsorption, thermodynamic and kinetic equations was carried out. The adsorption equilibrium was reached within 30 min, and the maximum adsorption capacity was 94.67 mg g-1 at 298 K. The adsorption capacity of nitrobenzene by the HKUST-1 shell is in good agreement with the Freundlich model and the second-order kinetic model. The possible mechanism of adsorption of nitrobenzene by the HKUST-1 shell was discussed. The experimental results suggested that Cu-BTC materials have potential applications for wastewater treatment involving benzene pollutants.

Dummy molecular imprinted polymers coated with silver microspheres via surface enhanced Raman scattering for sensitive detection of benzimidazole

Dummy molecular imprinted polymers (MIPs) with carbendazim as a dummy template coated with Ag microspheres were fabricated in N, N-dimethylformamide solution via a surface-enhanced Raman scattering (SERS) enhancement for detection of benzimidazole by using methylacrylamide and ethylene glycol dimethacrylate as the functional monomer and cross-linker, respectively. The scanning electron microscope, transmission electron microscope, energy dispersive spectrometer, particle size distribution analysis, and X-ray diffraction were used to perform the morphology, size, and crystal structure for prepared Ag@dummy MIPs. Under the optimal conditions, the SERS detection method presented good linearity with concentrations in the range of 1.0 × 10^-3 ~ 1.0 × 10^-8 mol L^-1 for benzimidazole at 774 cm^-1 and 1004 cm^-1, respectively. And the minimum detection concentration of this method was as low as 1.0 × 10^-8 mol L^-1. Besides, Ag@dummy MIPs exhibited satisfactory sensitivity and selectivity to benzimidazole instead of carbendazim due to the dummy imprinting technology to eliminate the background noise interference. The reusability result of Ag@dummy MIPs was shown that the characteristic peaks of benzimidazole are still obvious after four times of repeated detection. This method provided an effective way to develop a qualitative and semi-quantitative analysis of benzimidazole in complex matrices.

Strategies for SERS Detection of Organochlorine Pesticides

Organochlorine pesticides (OCPs) embody highly lipophilic hazardous chemicals that are being phased out globally. Due to their persistent nature, they are still contaminating the environment, being classified as persistent organic pollutants (POPs). They bioaccumulate through bioconcentration and biomagnification, leading to elevated concentrations at higher trophic levels. Studies show that human long-term exposure to OCPs is correlated with a large panel of common chronic diseases. Due to toxicity concerns, most OCPs are listed as persistent organic pollutants (POPs). Conventionally, separation techniques such as gas chromatography are used to analyze OCPs (e.g., gas chromatography coupled with mass spectrometry (GC/MS)) or electron capture detection (GC/ECD). These are accurate, but expensive and time-consuming methods, which can only be performed in centralized lab environments after extensive pretreatment of the collected samples. Thus, researchers are continuously fueling the need to pursue new faster and less expensive alternatives for their detection and quantification that can be used in the field, possibly in miniaturized lab-on-a-chip systems. In this context, surface enhanced Raman spectroscopy (SERS) represents an exceptional analytical tool for the trace detection of pollutants, offering molecular fingerprint-type data and high sensitivity. For maximum signal amplification, two conditions are imposed: an efficient substrate and a high affinity toward the analyte. Unfortunately, due to the highly hydrophobic nature of these pollutants (OCPs,) they usually have a low affinity toward SERS substrates, increasing the challenge in their SERS detection. In order to overcome this limitation and take advantage of on-site Raman analysis of pollutants, researchers are devising ingenious strategies that are synthetically discussed in this review paper. Aiming to maximize the weak Raman signal of organochlorine pesticides, current practices of increasing the substrate's performance, along with efforts in improving the selectivity by SERS substrate functionalization meant to adsorb the OCPs in close proximity (via covalent, electrostatic or hydrophobic bonds), are both discussed. Moreover, the prospects of multiplex analysis are also approached. Finally, other perspectives for capturing such hydrophobic molecules (MIPs-molecularly imprinted polymers, immunoassays) and SERS coupled techniques (microfluidics-SERS, electrochemistry-SERS) to overcome some of the restraints are presented.

An overview on molecular imprinted polymers combined with surface-enhanced Raman spectroscopy chemical sensors toward analytical applications

Surface-enhanced Raman spectroscopy (SERS) is a powerful and high-speed detection technology. It provides information on molecular fingerprint recognition with ultrahigh sensitive detection. However, it shows poor anti-interference capacity against complex matrices. Molecularly imprinted polymers (MIPs) can achieve specific recognition of targets from complex matrices. Through introducing the MIP separation system, the MIP-SERS chemical sensor can effectively overcome the limitation of complex matrix interference, and further improve the stability of sensors for detection. Herein, the materials and structures of integrated MIP-SERS sensors are systematically reviewed, and its application as a sensor for chemical detection of hazardous substances in environmental and food samples has been addressed as well. To broaden the prospects of application, we have discussed the current challenges and future perspectives that would accelerate the development of versatile MIP-SERS chemical sensors.

An efficient fluorescent nano-sensor of N-doped carbon dots for the determination of 2,4,6-trinitrophenol and other applications

N-Doped carbon dots (CDs) had been simply produced by a one-pot synthesis process using amygdalic acid and threonine. The resulting product was water-soluble and exhibited strong luminescence emission with a fluorescence quantum yield of 19.25%. The emission of CDs was obviously and selectively decreased upon adding 2,4,6-trinitrophenol (TNP). It was proved that the fluorescence resonance energy transfer was the main mechanism for quenching. An efficient fluorescence probe with satisfied sensitivity for TNP determination was found. The range of the linear response for TNP detection was 0.5-40.0 μmol L^-1, and the limit of detection was 20 nmol L^-1. The content of trace TNP in water samples was successfully detected with this method. The CDs were also applied in HepG2 cell imaging and the fabrication of fluorescent films by dispersing the solid freeze-drying CD (SCD) powder into PMMA, which exhibited some application value in biology and photovoltaics.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

Surface Imprinted Layer of Cypermethrin upon Au Nanoparticle as a Specific and Selective Coating for the Detection of Template Pesticide Molecules

The detection of specific pesticides on food products is essential as these substances pose health risks due to their toxicity. The use of surface-enhanced Raman spectroscopy (SERS) takes advantage of the straightforward technique to obtain fingerprint spectra of target analytes. In this study, SERS-active substrates are made using Au nanoparticles (NPs) coated with a layer of polymer and followed by imprinting with a pesticide–Cypermethrin, as a molecularly imprinted polymer (MIP). Cypermethrin was eventually removed and formed as template cavities, then denoted as Au NP/MIP, to capture the analogous molecules. The captured molecules situated in-between the areas of high electromagnetic field formed by plasmonic Au NPs result in an effect of SERS. The formation of Au NP/MIP was, respectively, studied through morphological analysis using transmission electron microscopy (TEM) and compositional analysis using X-ray photoelectron spectroscopy (XPS). Two relatively similar pesticides, Cypermethrin and Permethrin, were used as analytes. The results showed that Au NP/MIP was competent to detect both similar molecules despite the imprint being made only by Cypermethrin. Nevertheless, Au NP/MIP has a limited number of imprinted cavities that result in sensing only low concentrations of a pesticide solution. Au NP/MIP is thus a specific design for detecting analogous molecules similar to its template structure.

A novel SERS selective detection sensor for trace trinitrotoluene based on meisenheimer complex of monoethanolamine molecule

Trinitrotoluene (TNT) is a primary component in chemical explosives, making them a common focus in public safety detection. However, it is very difficult to achieve selective and sensitive detection of the TNT molecule in practical application. In the present study, a simple surface enhanced Raman scattering (SERS) sensing based on monoethanolamine (MEA) - modified gold nanoparticles (Au NPs) was expanded for high selectivity and sensitive detecting of TNT in an envelope, luggage, lake water, and clothing through a quickly sampling and detection process. The monoethanolamine molecule based on Meisenheimer complex lights up ultra-high Raman scattering of a nonresonant molecule on the superficial coat of gold nanoparticles. Using this detection sensor, a molecular bridge can be established to selectively detect trinitrotoluene with a detection limit of 21.47 pM. We were able to rapidly identification trinitrotoluene molecule with a powerful selective over the familiar interfering substances nitrophenol, picric acid, 2,4-dinitrophenol, and 2,4-dinitrotoluene. The outcome in this work supply an efficient solution to the test of trinitrotoluene and to establishing a SERS sensor analytical strategy. The studies have demonstrated that the MEA-Au NPs based SERS sensing can be potentially used in field detection the trace amount of chemical explosives for public security.

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.

SERS-Based Molecularly Imprinted Plasmonic Sensor for Highly Sensitive PAH Detection

A novel hybrid plasmonic platform based on the synergetic combination of a molecularly imprinted polymer (MIP) thin film with Au nanoparticle (NPs) assemblies, noted as Au@MIP, was developed for surface-enhanced Raman scattering (SERS) spectroscopy recognition of polycyclic aromatic hydrocarbons (PAHs). While the MIP trapped the PAH close to the Au surface, the plasmonic NPs enhanced the molecule's Raman signal. The Au@MIP fabrication comprises a two-step procedure, first, the layer-by-layer deposition of Au NPs on glass and their further coating with a uniform MIP thin film. Profilometry analysis demonstrated that the thickness and homogeneity of the MIP film could be finely tailored by tuning different parameters such as prepolymerization time or spin-coating rate. Two different PAH molecules, pyrene or fluoranthene, were used as templates for the fabrication of pyrene- or fluoranthene-based Au@MIP substrates. The use of pyrene or fluoranthene, as the template molecule to fabricate the Au@MIP thin films, enabled its ultradetection in the nM regime with a 100-fold improvement compared with the nonimprinted plasmonic sensors (Au@NIPs). The SERS data analysis allowed to estimate the binding constant of the template molecule to the MIP. The selectivity of both pyrene- and fluoranthene-based Au@MIPs was analyzed against three PAHs of different sizes. The results displayed the important role of the template molecule used for the Au@MIPs fabrication in the selectivity of the system. Finally, the practical applicability of pyrene-based Au@MIPs was shown by performing the detection of pyrene in two real samples: creek water and seawater. The design and optimization of this type of plasmonic platform will pave the way for the detection of other relevant (bio)molecules in a broad range of fields such as environmental control, food safety, or biomedicine.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Direct SERDS sensing of molecular biomarkers in plants under field conditions

Molecular biomarkers such as microRNAs (miRNAs) play important roles in regulating various developmental processes in plants. Understanding these pathways will help bioengineer designing organisms for efficient biomass accumulation. Current methods for RNA analysis require sample extraction and multi-step sample analysis, hindering work in field studies. Recent work in the incorporation of nanomaterials for plant bioengineering research is leading the way of an agri-tech revolution. As an example, surface-enhanced Raman scattering (SERS)-based sensors can be used to monitor RNA in vivo. However, the use of SERS in the field has been limited due to issues with observing Raman signal over complex background. To this end, shifted-excitation Raman difference spectroscopy (SERDS) offers an effective solution to extract the SERS signal from high background based on a physical approach. In this manuscript, we report the first application of SERDS on SERS sensors. We investigated this technique on SERS sensor developed for the detection of a microRNA biomarker, miR858. We tested the technique on in vitro samples and validated the technique by detecting the presence of exogenous miR858 in plants directly under ambient light in a growth chamber. The possibility of moving the detection of nucleic acid targets outside the constraints of laboratory setting enables numerous important bioengineering applications. Such applications can revolutionize biofuel production and agri-tech through the use of nanotechnology-based monitoring of plant growth, plant health, and exposure to pollution and pathogens.

Phenanthroimidazole derivatives as a chemosensor for picric acid: a first realistic approach

A series of phenanthroimidazole (PI) derivatives (M1–M3): 2-phenyl-1H-phenanthro [9,10-d]imidazole (M1), 2-anthryl-1H-phenanthro[9,10-d]imidazole (M2), and 2-pyrenyl-1H-phenanthro[9,10-d]imidazole (M3) were synthesized and characterized using various spectroscopic techniques.

Surface-enhanced Raman scattering-based detection of hazardous chemicals in various phases and matrices with plasmonic nanostructures

Surface-enhanced Raman scattering (SERS)-based sensors utilize the electromagnetic-field enhancement of plasmonic substrates with the chemical specificity of vibrational Raman spectroscopy to identify trace amounts of a wide variety of different target analytes while being minimally affected by photobleaching. However, despite many advantageous features of this method, SERS sensors, particularly for detecting hazardous chemicals, suffer from several limitations such as requirement of gigantic signal enhancement that is often poorly controllable, subtle change and degradation of the SERS substrate, consecutive fluctuation of the signal, the lack of reliable receptors for capturing targets of interest and the absence of general principles for detecting various chemicals in different phases and matrices. To overcome these limitations and for SERS sensors to find practical use, one must (1) acknowledge the characteristics of the matrices of target systems, (2) finely engineer and tune the receptors of the SERS sensor to properly extract the target analyte from the phase, and (3) implement additional mechanistic modifications to enhance the plasmonic signal. This minireview underlines the difficulties associated with different phases and a wide range of target analytes, and introduces the practical measures undertaken to overcome the respective difficulties in SERS-based detection of hazardous chemicals.

Molecularly Imprinted Polymer Materials as Selective Recognition Sorbents for Explosives: A Review

Explosives are of significant interest to homeland security departments and forensic investigations. Fast, sensitive and selective detection of these chemicals is of great concern for security purposes as well as for triage and decontamination in contaminated areas. To this end, selective sorbents with fast binding kinetics and high binding capacity, either in combination with a sensor transducer or a sampling/sample-preparation method, are required. Molecularly imprinted polymers (MIPs) show promise as cost-effective and rugged artificial selective sorbents, which have a wide variety of applications. This manuscript reviews the innovative strategies developed in 57 manuscripts (published from 2006 to 2019) to use MIP materials for explosives. To the best of our knowledge, there are currently no commercially available MIP-modified sensors or sample preparation methods for explosives in the market. We believe that this review provides information to give insight into the future prospects and potential commercialization of such materials. We warn the readers of the hazards of working with explosives.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Plasmonic Nanoprobes for in Vivo Multimodal Sensing and Bioimaging of MicroRNA within Plants

Monitoring gene expression within whole plants is critical for many applications ranging from plant biology to agricultural biotechnology and biofuel development; however, no method currently exists for in vivo monitoring of genomic targets in plant systems without requiring sample extraction. Herein, we report a unique multimodal method based on plasmonic nanoprobes capable of in vivo imaging and biosensing of microRNA biotargets within whole plant leaves by integrating three different and complementary techniques: surface-enhanced Raman scattering (SERS), X-ray fluorescence (XRF), and plasmonics-enhanced two-photon luminescence (TPL). The method developed uses plasmonic nanostars, which not only provide large Raman signal enhancement but also allow for localization and quantification by XRF and plasmonics-enhanced TPL, owing to gold content and high two-photon luminescence cross sections. Our method uses inverse molecular sentinel nanoprobes for SERS bioimaging of microRNA within Arabidopsis thaliana leaves to provide a dynamic SERS map of detected microRNA targets while also quantifying nanoprobe concentrations using XRF and TPL. The nanoprobes were observed to occupy the intercellular spaces upon infiltration into the leaf tissues. This report lays the foundation for the use of plasmonic nanoprobes for in vivo functional imaging of nucleic acid biotargets in whole plants, a tool that will revolutionize bioengineering research by allowing the study of these biotargets with previously unmet spatial and temporal resolution, 200 μm and 30 min, respectively.

Green Synthesis of Blue Fluorescent P-doped Carbon Dots for the Selective Determination of Picric Acid in an Aqueous Medium

A fluorescence method for the determination of picric acid (PA) using phosphorus-doped carbon dots (P-CDs), synthesized from β-cyclodextrin and sodium pyrophosphate, is described. The P-CDs are very uniform and monodisperse with a diameter of about 2.8 nm. Under an excitation of 350 nm, the P-CDs emit bright blue fluorescence with an emission peak at 440 nm. The as-synthesized P-CDs serve as a sensitive, selective, and label-free fluorescent probe for the detection of PA. Based on an inner filter effect between PA and P-CDs, a linear response is obtained for PA from 0.1 to 10 μM with a detection limit of 82 nM. Finally, this sensing system has been demonstrated to have practicability for PA detection in the environmental water samples.

Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation. Initially, pure Ag and Cu NPs were produced individually using the laser ablation in liquid technique (with ∼50 femtosecond pulses at 800 nm) which was followed by laser irradiation of the mixed Ag and Cu NPs in equal volume. These Ag, Cu, and Ag–Cu NPs were characterised by UV-visible absorption, HRTEM and XRD techniques. The alloy formation was confirmed by the presence of a single surface plasmon resonance peak in absorption spectra and elemental mapping using FESEM techniques. Furthermore, the results from surface enhanced Raman scattering (SERS) studies performed for the methylene blue (MB) molecule suggested that Ag–Cu alloy NPs demonstrate a higher enhancement factor (EF) compared to pure Ag/Cu NPs. Additionally, SERS studies of Ag–Cu alloy NPs were implemented for the detection of explosive molecules such as picric acid (PA – 5 μM), ammonium nitrate (AN – 5 μM) and the dye molecule methylene blue (MB – 5 nM). These alloy NPs exhibited superiority in the detection of various analyte molecules with good reproducibility and high sensitivity with EFs in the range of 10⁴ to 10⁷.

Herein we demonstrate the synthesis of Ag–Cu alloy NPs through a consecutive two-step process; laser ablation followed by laser irradiation.

Manganese-doped carbon quantum dots-based fluorescent probe for selective and sensitive sensing of 2,4,6-trinitrophenol via an inner filtering effect

In the present work, a selective and sensitive method for detecting TNP using manganese doped carbon quantum dots (Mn-CDs) was developed. The Mn-CDs were prepared via a simple hydrothermal method using 1-(2-pyridinylazo)-2-naohthalenol naohthalenol (PAN) and MnCl₂ as precursors. The as-prepared Mn-CDs have UV emission with high quantum yield (83.2%). Because of the strong characteristic absorption of TNP at 356 nm, which has good spectral overlap with the emission peak of Mn-CDs, the fluorescence intensity of Mn-CDs at 360 nm is linearly quenched in the presence of TNP in the concentration range of 0.1-200 μM. The developing assay based on an inner filter effect (IFE) mechanism for detecting TNP is selective, convenient, and shows that the as-prepared Mn-CDs have application prospects for simple and specific analytical chemistry.

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

A glassy carbon electrode with electrodeposited silver nanoparticles for aptamer based voltammetric determination of trinitrotoluene using riboflavin as a redox probe

An electrochemical nanoaptasensor is described that is based on the use of a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs). An aptamer (Apt) against trinitrotoluene (TNT) was then immobilized on the AgNPs. The addition of TNT to the modified GCE leads to decrease in peak current (typically measured at a potential of -0.45 V vs. Ag/AgCl) of riboflavin which acts as an electrochemical probe. Even small changes in the surface (as induced by binding of Apt to TNT) alter the interfacial properties. As a result, the LOD is lowered to 33 aM, and the dynamic range extends from 0.1 fM to 10 μM without sacrificing specificity. Graphical abstract Schematic presentation of a nanoaptasensor which is based on a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs) and aptamer (Apt). It was applied to the detection of 2,4,6-trinitrotoluene (TNT) with the help of riboflavin (RF) as a redox probe.

Molecular Affinity Agents for Intrinsic Surface-Enhanced Raman Scattering (SERS) Sensors

Research at the interface of synthetic materials, biochemistry, and analytical techniques has enabled sensing platforms for applications across many research communities. Herein we review the materials used as affinity agents to create surface-enhanced Raman spectroscopy (SERS) sensors. Our scope includes those affinity agents (antibody, aptamer, small molecule, and polymer) that facilitate the intrinsic detection of targets relevant to biology, medicine, national security, environmental protection, and food safety. We begin with an overview of the analytical technique (SERS) and considerations for its application as a sensor. We subsequently describe four classes of affinity agents, giving a brief overview on affinity, production, attachment chemistry, and first uses with SERS. Additionally, we review the SERS features of the affinity agents, and the analytes detected by intrinsic SERS with that affinity agent class. We conclude with remarks on affinity agent selection for intrinsic SERS sensing platforms.

Electrospun flexible poly(bisphenol A carbonate) nanofibers decorated with Ag nanoparticles as effective 3D SERS substrates for trace TNT detection

A flexible 3D hybrid PC/Ag surface-enhanced Raman scattering (SERS) substrate was fabricated through the combination of electrospinning and in situ chemical reduction. Due to the rough surface morphology and the intricate 3D structure, a high density of Raman "hotspots" was formed at the junctions of cross-linked nanofibers, resulting in excellent sensitivity to a probe molecule (4-aminothiophenol). The nanofibers were modified with l-cysteine to capture TNT molecules by the formation of a Meisenheimer complex, after which positively charged 4-ATP-labelled AgNPs were introduced to the system, which both generated more hotspots and led to a linear relationship between the TNT concentration and the SERS intensity of the labelled molecules. As a result, a good linear response ranging from 10^-8 to 10^-12 M was achieved, and the detection limit for TNT was as low as 2.05 × 10^-13 M. This strategy demonstrates an ultra-sensitive approach for the detection of trace amounts of TNT, and a promising method for the detection of many other analogous explosives.

Fabrication of a Flexible Gold Nanorod Polymer Metafilm via a Phase Transfer Method as a SERS Substrate for Detecting Food Contaminants

Surface enhanced Raman scattering (SERS) has been widely used in detection of food safety due to the nondestructive examination property. Here, we reported a flexible SERS film based on a polymer-immobilized gold nanorod polymer metafilm. Polystyrene-polyisoprene-polystyrene (SIS), a transparent and flexible, along with having excellent elasticity, polymer, was chosen as the main support of gold nanorods. A simple phase transfer progress was adopted to mix the gold nanorods with the polymer, which can further be used in most water-insoluble polymers. The SERS film performed satisfactorily while being tested in a series of standard Raman probes, like crystal violet (CV) and malachite green (MG). Moreover, the excellent reproducibility and elastic properties make the film a promising substrate in practical detection. Hence, the MG detection on the fish surface and trace thiram detection on orange pericarp were inspected with detection results of 1 × 10^-10 and 1 × 10^-6 M, which were below the demand of the National standard of China, exactly matching the realistic application requirements.

Fluorescence chemical sensor for determining trace levels of nitroaromatic explosives in water based on conjugated polymer with guanidinium side groups

A novel fluorescent conjugated polymer (poly(2-amino-N-(2-((4-ethynylphenyl) ethynyl) phenyl)-5-guanidinopentanamide)-1,4-phenylethynylene-1,4-phenyleneethynylene, PPE-Arg) was synthesized in this paper. We found that PPE-Arg could be quenched by picric acid (PA). Photoinduced electron transfer (PET) mechanism can be used to describe the fluorescence quenching of PPE-Arg. It could be speculated that the photo-induced electrons may be transferred from PPE-Arg to nitroaromatic explosives. In this paper, the experiment conditions and detection performance of PPE-Arg were systematically studied. The experiment results demonstrate PPE-Arg as a sensor for PA has a good linear range from 5 × 10^-7 to 6 × 10^-5 mol L^-1 with the calculated limit of detection (LOD) to be 1.0 × 10^-7 mol L^-1. Meanwhile, reaction time between PPE-Arg and PA is less than 1 min. This proposed sensor was applied to rapidly detect nitroaromatic explosives in environmental water samples and satisfactory results were obtained.