Detection and Quantitation of Trace Fentanyl in Heroin by Surface-Enhanced Raman Spectroscopy

Femtosecond laser-induced Au nanostructure-decorated with plasmonic nanomaterials for sensitive SERS-based detection of fentanyl

Fentanyl and its analogs have emerged as the main factor behind the ongoing opioid abuse globally in recent years. However, the existing techniques for sensitive and accurate detection of fentanyl are often complex, laborious, expensive, and restricted to central healthcare facilities. We reported herein a plasmonic biochip fabricated by the femtosecond laser-induced nanostructures and plasmonic nanomaterials for sensitive SERS-based detection of fentanyl. Yolk-shell structured plasmonic nanomaterials are employed owing to their unique optical properties. The femtosecond laser direct writing technique creates three-dimensional silicon nanostructures followed by gold deposition and the immobilization of plasmonic nanomaterials. This SERS biochip fabricated by the femtosecond laser-induced nanostructure decorated with yolk-shell structured plasmonic nanomaterials enables the rapid and sensitive detection of fentanyl with the limit of detection of 3.33 ng/mL.

Optimized machine learning approaches to combine surface-enhanced Raman scattering and infrared data for trace detection of xylazine in illicit opioids

Infrared absorption spectroscopy and surface-enhanced Raman spectroscopy were integrated into three data fusion strategies-hybrid (concatenated spectra), mid-level (extracted features from both datasets) and high-level (fusion of predictions from both models)-to enhance the predictive accuracy for xylazine detection in illicit opioid samples. Three chemometric approaches-random forest, support vector machine, and k-nearest neighbor algorithms-were employed and optimized using a 5-fold cross-validation grid search for all fusion strategies. Validation results identified the random forest classifier as the optimal model for all fusion strategies, achieving high sensitivity (88% for hybrid, 92% for mid-level, and 96% for high-level) and specificity (88% for hybrid, mid-level, and high-level). The enhanced performance of the high-level fusion approach (F1 score of 92%) is demonstrated, effectively leveraging the surface-enhanced Raman data with a 90% voting weight, without compromising prediction accuracy (92%) when combined with infrared spectral data. This highlights the viability of a multi-instrument approach using data fusion and random forest classification to improve the detection of various components in complex opioid samples in a point-of-care setting.

Factors that Affect Quantification in Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an analytical technique capable of detecting trace amounts of specific species. The uniqueness of vibrational signatures is a major advantage of SERS. This combination of sensitivity and specificity has motivated researchers to develop diverse analytical methodologies leveraging SERS. However, even 50 years after its first observation, SERS is still perceived as an unreliable technique for quantification. This perception has precluded the application of SERS in laboratories that rely on consistent quantification (for regulatory purposes, for instance). In this review, we describe some of the aspects that lead to SERS intensity variations and how those challenges were addressed in the 50 years of the technique. The goal is to identify the sources of variations in SERS intensities and then demonstrate that, even with these pitfalls, the technique can be used for quantification when factors such as nature of the substrate, experimental conditions, sample preparation, surface chemistry, and data analysis are carefully considered and tailored for a particular application.

Detection of low-level fentanyl concentrations in mixtures of cocaine, MDMA, methamphetamine, and caffeine via surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) was utilized to measure low-level fentanyl concentrations mixed in common cutting agents, cocaine, 3,4-methylenedioxymethamphetamine (MDMA), methamphetamine, and caffeine. Mixtures were prepared with a fentanyl concentration range of 0-339 μM. Data was initially analyzed by plotting the area of a diagnostic peak (1026 cm-1) against concentration to generate a calibration model. This method was successful with fentanyl/MDMA samples (LOD 0.04 μM) but not for the other mixtures. A chemometric approach was then employed. The data was evaluated using principal component analysis (PCA), partial least squares (PLS1) regression, and linear discriminant analysis (LDA). The LDA model was used to classify samples into one of three designated concentration ranges, low = 0-0.4 mM, medium = 0.4-14 mM, or high >14 mM, with fentanyl concentrations correctly classified with greater than 85% accuracy. This model was then validated using a series of "blind" fentanyl mixtures and these unknown samples were assigned to the correct concentration range with an accuracy >95%. The PLS1 model failed to provide accurate quantitative assignments for the samples but did provide an accurate prediction for the presence or absence of fentanyl. The combination of the two models enabled accurate quantitative assignment of fentanyl in binary mixtures. This work establishes a proof of concept, indicating a larger sample size could generate a more accurate model. It demonstrates that samples, containing variable, low concentrations of fentanyl, can be accurately quantified, using SERS.

Ultrasensitive electrochemiluminescencent multivalent aptamer sensor based on energy and electron transfer dual quenching tactics

An efficient "on-off-on" electrochemiluminescence (ECL) aptasensor utilizing dual-mechanism quenching was constructed for detecting furanyl fentanyl (FuF). The first signal "on" state was achieved by novel dual-ligand zinc metal-organic frameworks (Zn-MOFs), which were synthesized by self-assembly reaction using zinc atom clusters as metal nodes, achieving strong and stable ECL emission. The "off" state was realized by the energy and electron quenching effect of copper-doped WO3. Specifically, in addition to the overlap of the UV-Vis spectrum, energy transfer existed between the acceptor and donor after doping copper. Therefore, copper created a "highway" for electron transfer between the donor and acceptor, which greatly improved the quenching efficiency. Simultaneously, employing multivalent aptamers as capture probes augmented the binding affinity and probability of association between aptamer and target through a synergistic multivalent effect. Consequently, the second "on" state was the ECL signal restored by introducing FuF. Benefiting from the combination of the multivalent aptamers strategy with the "on-off-on" design, the developed aptasensor showed excellent linearity (1.0 × 10-13 to 1.0 × 10-6 g/L) with a low limit of detection of 5.5 × 10-14 g/L (S/N = 3). Additionally, it demonstrates a relative standard deviation (RSD) of less than 5% and good recovery ranging from 97.6 to 102%. The proposed aptasensor presents considerable potential for rapid, sensitive, and accurate determination of FuF.

A flexible plasmonic substrate for sensitive surface-enhanced Raman scattering-based detection of fentanyl

In this work, we demonstrate a straightforward and versatile approach for fabricating flexible SERS substrates for highly sensitive fentanyl detection. Our design strategy integrates the synthesis of a yolk-shell structured plasmonic nanomaterial with a flexible cellulose substrate. The resulting SERS platform demonstrates excellent sensing capabilities, achieving a fentanyl detection limit as low as 4.89 ng mL-1.

Combining surface-enhanced Raman spectroscopy (SERS) and paper spray mass spectrometry (PS-MS) for illicit drug detection

There is an ongoing effort in the US illicit drug market to make new psychoactive compounds more potent and addictive. Due to continuous chemical modifications, many fentanyl analogs are developed and mixed with more traditional illicit drugs, such as cocaine and heroin. Detecting fentanyl and fentanyl analogs in these illicit drug mixtures has become more crucial because of the increased potency and associated health risks. Most confirmatory procedures require time-consuming and expensive, highly sophisticated laboratory equipment and experimental procedures, which can delay critical information that might save a victim or find a suspect. In this study, we propose miniaturizing and accelerating this process by combining surface-enhanced Raman spectroscopy (SERS) analysis and paper spray mass spectrometry (PS-MS). For this aim, dual-purposed paper substrates were developed through soaking in Au/Ag nanostars suspensions. These novel, in-house prepared paper SERS substrates showed stability for up to four weeks with and without the presence of drug compounds. Fentanyl analogs with similar SERS spectra were differentiated by coupling with PS-MS. The limit of detection (LOD) for fentanyl on the paper substrates is 34 μg/mL and 0.32 μg/mL for SERS and PS-MS, respectively. Fentanyl and fentanyl analogs show selective SERS enhancement that helped to detect trace amounts of these opioids in heroin and cocaine street samples. In short, we propose the combination of SERS/PS-MS by using modified paper substrates to develop cost-effective, sensitive, rapid, portable, reliable, and reproducible methods to detect illicit drugs, especially trace amounts of fentanyl and fentanyl analogs in illicit drug mixtures. The combination of these two category A techniques allows for the identification of illicit drugs according to the SWGDRUG guidelines.

Mitigating Medication Tampering and Diversion via Real-Time Intravenous Opioid Quantification

Opioid tampering and diversion pose a serious problem for hospital patients with potentially life-threatening consequences. The ongoing opioid crisis has resulted in medications used for pain management and anesthesia, such as fentanyl and morphine, being stolen, substituted with a different substance, and abused. This work aims to mitigate tampering and diversion through analytical verification of the administered drug before it enters the patient. We present an electrochemical-based sensor and miniaturized wireless potentiostat that enable real-time intravenous (IV) monitoring of opioids, specifically fentanyl and morphine. The proposed system is connected to an IV drip system during surgery or post-operation recovery. Measurement results of two opioids are presented, including calibration curves and data on the sensor performance concerning pH, temperature, interference, reproducibility, and long-term stability. Finally, we demonstrate real-time fluidic measurements connected to a flow cell to simulate IV administration and a blind study classified using a machine-learning algorithm. The system achieves limits of detection (LODs) of 1.26 µg/mL and 2.75 µg/mL for fentanyl and morphine, respectively, while operating with >1-month battery lifetime due to an optimized ultra-low power 36 µA sleep mode.

Trace Detection of Adulterants in Illicit Opioid Samples Using Surface-Enhanced Raman Scattering and Random Forest Classification

The detection of trace adulterants in opioid samples is an important aspect of drug checking, a harm reduction measure that is required as a result of the variability and unpredictability of the illicit drug supply. While many analytical methods are suitable for such analysis, community-based approaches require techniques that are amenable to point-of-care applications with minimal sample preparation and automated analysis. We demonstrate that surface-enhanced Raman spectroscopy (SERS), combined with a random forest classifier, is able to detect the presence of two common sedatives, bromazolam (0.32-36% w/w) and xylazine (0.15-15% w/w), found in street opioid samples collected as a part of a community drug checking service. The Raman predictions, benchmarked against mass spectrometry results, exhibited high specificity (88% for bromazolam, 96% for xylazine) and sensitivity (88% for bromazolam, 92% for xylazine) for the compounds of interest. We additionally provide evidence that this exceeds the performance of a more conventional approach using infrared spectral data acquired on the same samples. This demonstrates the feasibility of SERS for point-of-care analysis of challenging multicomponent samples containing trace adulterants.

Label-free SERS-ML detection of cocaine trace in human blood plasma

The widespread consumption of cocaine poses a significant threat to modern society. The most effective way to combat this problem is to control the distribution of cocaine, based on its accurate and sensitive detection. Here, we proposed the detection of cocaine in human blood plasma using a combination of surface enhanced Raman spectroscopy and machine learning (SERS-ML). To demonstrate the efficacy of our proposed approach, cocaine was added into blood plasma at various concentrations and drop-deposited onto a specially prepared disposable SERS substrate. SERS substrates were created by deposition of metal nanoclusters on electrospun polymer nanofibers. Subsequently, SERS spectra were measured and as could be expected, the manual distinguishing of cocaine from the spectra proved unfeasible, as its signal was masked by the background signal from blood plasma molecules. To overcome this issue, a database of SERS spectra of cocaine in blood plasma was collected and used for ML training and validation. After training, the reliability of proposed approach was tested on independently prepared samples, with unknown for SERS-ML cocaine presence or absence. As a result, the possibility of rapid determination of cocaine in blood plasma with a probability above 99.5% for cocaine concentrations up to 10-14 M was confirmed. Therefore, it is evident that the proposed approach has the ability to detect trace amounts of cocaine in bioliquids in an express and simple manner.

Differentiating Structurally Similar Fentanyl Analogs by Comparing Density Functional Theory (DFT) Calculations and Surface-Enhanced Raman Spectroscopy (SERS) Results

Fentanyl and fentanyl analogs are the main cause of recent overdose deaths in the United States. The presence of fentanyl analogs in illicit drugs makes it difficult to estimate their potencies. This makes the detection and differentiation of fentanyl analogs critically significant. Surface-enhanced Raman spectroscopy (SERS) can differentiate structurally similar fentanyl analogs by yielding spectroscopic fingerprints for the detected molecules. In previous years, five fentanyl analogs, carfentanil, furanyl fentanyl, acetyl fentanyl, 4-fluoroisobutyryl fentanyl (4-FIBF), and cyclopropyl fentanyl (CPrF), gained popularity and were found in 76.4% of the fentanyl analogs trafficked. In this study, we focused on 4-FIBF, CPrF, and structurally similar fentanyl analogs. We developed methods to differentiate these fentanyl analogs using theoretical and experimental methods. To do this, a set of fentanyl analogs were examined using density functional theory (DFT) calculations. The DFT results obtained in this project permitted the assignment of spectral bands. These results were then compared with normal Raman and SERS techniques. Structurally similar fentanyl analogs show important differences in their spectra, and they have been visually differentiated from each other both theoretically and experimentally. Additional results using principal component analysis and soft independent modeling of class analogy show they can be distinguished using this technique. The limit of detection values for FIBF and CPrF were determined to be 0.35 ng/mL and 4.4 ng/mL, respectively, using SERS. Experimental results obtained in this project can be readily implemented in field applications and smaller laboratories, where inexpensive portable Raman spectrometers are often present and used in drug analysis.

Microfluidic sensors for the detection of emerging contaminants in water: A review

In recent years, numerous emerging contaminants have been identified in surface water, groundwater, and drinking water. Developing novel sensing methods for detecting diverse emerging pollutants in water is urgently needed, as even at low concentrations, these pollutants can pose a serious threat to human health and environmental safety. Traditional testing methods are based on laboratory equipment, which is highly sensitive but complex to operate, costly, and not suitable for on-site monitoring. Microfluidic sensors offer several benefits, including rapid evaluation, minimal sample usage, accurate liquid manipulation, compact size, automation, and in-situ detection capabilities. They provide promising and efficient analytical tools for high-performance sensing platforms in monitoring emerging contaminants in water. In this paper, recent research advances in microfluidic sensors for the detection of emerging contaminants in water are reviewed. Initially, a concise overview is provided about the various substrate materials, corresponding microfabrication techniques, different driving forces, and commonly used detection techniques for microfluidic devices. Subsequently, a comprehensive analysis is conducted on microfluidic detection methods for endocrine-disrupting chemicals, pharmaceuticals and personal care products, microplastics, and perfluorinated compounds. Finally, the prospects and future challenges of microfluidic sensors in this field are discussed.

Electrochemical Determination of Fentanyl Using Carbon Nanofiber-Modified Electrodes

In this work, we report the direct electrochemical oxidation of fentanyl using commercial screen-printed carbon electrodes (SPCEs) modified with carboxyl-functionalized carbon nanofibers (fCNFs). CNFs have surface chemistry and reactivity similar to carbon nanotubes (CNTs), yet they are easier to produce and are of a lower cost than CNTs. By monitoring the current produced during the electrochemical oxidation of fentanyl, variables such as fCNF loading, fentanyl accumulation time, electrolyte pH, and differential pulse voltammetry parameters were optimized. Under an optimized set of conditions, the fCNF/SPCEs responded linearly to fentanyl in the concentration range of 0.125-10 μM, with a limit of detection of 75 nM. The fCNF/SPCEs also demonstrated excellent selectivity against common cutting agents found in illicit drugs (e.g., glucose, sucrose, caffeine, acetaminophen, and theophylline) and interferents found in biological samples (e.g., ascorbic acid, NaCl, urea, creatinine, and uric acid). The performance of the sensor was also successfully tested using fentanyl spiked into an artificial urine sample. The straightforward electrode assembly process, low cost, ease of use, and rapid response make the fCNF/SPCEs prime candidates for the detection of fentanyl in both physiological samples and street drugs.

Developing Aptamer-Based Colorimetric Opioid Tests

Opioids collectively cause over 80,000 deaths in the United States annually. The ability to rapidly identify these compounds in seized drug samples on-site will be essential for curtailing trafficking and distribution. Chemical reagent-based tests are fast and simple but also notorious for giving false results due to poor specificity, whereas portable Raman spectrometers have excellent selectivity but often face interference challenges with impure drug samples. In this work, we develop on-site sensors for morphine and structurally related opioid compounds based on in vitro-selected oligonucleotide affinity reagents known as aptamers. We employ a parallel-and-serial selection strategy to isolate aptamers that recognize heroin, morphine, codeine, hydrocodone, and hydromorphone, along with a toggle-selection approach to isolate aptamers that bind oxycodone and oxymorphone. We then utilize a new high-throughput sequencing-based approach to examine aptamer growth patterns over the course of selection and a high-throughput exonuclease-based screening assay to identify optimal aptamer candidates. Finally, we use two high-performance aptamers with KD of ∼1 μM to develop colorimetric dye-displacement assays that can specifically detect opioids like heroin and oxycodone at concentrations as low as 0.5 μM with a linear range of 0-16 μM. Importantly, our assays can detect opioids in complex chemical matrices, including pharmaceutical tablets and drug mixtures; in contrast, the conventional Marquis test completely fails in this context. These aptamer-based colorimetric assays enable the naked-eye identification of specific opioids within seconds and will play an important role in combatting opioid abuse.

Recent reports on the sensing strategy and the On-site detection of illegal drugs

In this review, works on the on-site detection of illegal drugs in recent years are summarised and discussed, most of which were published within the past five years. The detection methods are categorised as colourimetric, fluorescence, Raman spectrometry, ion mobility spectrometry, electrochemistry, and mass spectrometry. Also, strategies that are possibly suitable for on-site detection and the actual instrumentation to be used in the field are listed.

Silica monolayer isolation of electrodeposited SERS substrate and online probing of molecule adsorption onto mineral microparticles in relation to flotation

In the minerals processing industry, the surface chemistry of mineral particles and its real-time detection can significantly enhance process performance, and ultimately leading to automotive and intelligent control. The adsorption of collector molecule onto bulk mineral specimens could be investigated with the help of shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). However, this method is unsuitable for the online detection of particles fluid consisted of micro-sized chalcocite that encountered in industrial production processes. In this work, a novel strategy of shell-isolated nanoparticles synthesis by electrodeposition of gold nanoparticles film and isolation of this film with crosslinked silica monolayer was proposed. The adsorption of 2-mercaptobenzothiazole (MBT), a typical flotation collector, onto a copper sulfide mineral, chalcocite was measured in-situ with the help of such a SERS substrate. Enhancement factors of 106-107 was calculated based on an idealized model. Furthermore, we discussed the stability of the silica isolation monolayer under high-power laser irradiation.

Light-induced synthesis of silver nanoprisms as a surface-enhanced Raman scattering substrate for N-acetyl procainamide drug quantification

Triangle-shaped silver nanoprisms (AgNPMs) were prepared by a photo-induced method through a seed-mediated growth process and were successfully employed as an ultra-sensitive surface-enhanced Raman scattering (SERS) substrate for the detection of the chemotherapeutic N-acetyl procainamide (NAPA) compound. The transformation of the morphology of the nanoprisms substrate could be noted with a remarkable change in color, possessing an average size of 95 nm. The shape-modified AgNPMs exhibited interesting optical characteristics owing to the truncated dual edges, which led to a pronounced longitudinal localized surface plasmonic resonance (LLSPR) behavior. The nanoprisms-based SERS substrate demonstrated an outstanding sensitivity for NAPA in aqueous solutions with the lowest ever reported detection limit of 0.5 × 10-13 M corresponding to excellent recovery and stability. A steady linear response with a broad dynamic range (10-4-10-12 M) and an R2 of 0.945 was also achieved. The results proved that the NPMs demonstrated excellent efficiency, reproducibility (97%), and stability (30 days) with a superior Raman signal enhancement reaching an ultralow detection limit of 0.5 × 10-13 M compared to the nanosphere particles which could show an LOD of 0.5 × 10-9 M.

Stepwise Au decoration-assisted double signal amplified lateral flow strip for ultrasensitive detection of morphine in fingerprint sweat

Point-of-care testing (POCT) of morphine (MOP) without invasion of privacy is of critical importance for law-enforcement departments to realize on-site rapid screening. In this study, ultrasensitive and non-invasive screening of MOP residues in the fingerprint sweat was easily realized by stepwise Au decoration-assisted double signal amplification and antibody-saving strategies on lateral flow strip (LFS). The construction of LFS was not intrinsically changed compared with traditional LFS except the labeling material on conjugation pad for enhanced signal reporting. The gold nanoparticle-seeded SiO2 was adopted as the labeling materials in place of traditional gold nanoparticles, which acted as the first-round signal amplification and ready for second-round gold deposition-assisted amplification. And the second-round amplification could be completed in just 10 s, which did not alter the intrinsic simplicity of LFS for rapid and on-site screening. With the designed signal amplification principle of LFS, target MOP in the fingerprint sweat can be effectively transferred to the LFS for analysis without invasion of privacy. As low as 0.5 pg MOP in fingerprint sweat can be visually judged with this double signal amplified LFS, the sensitivity of which has been improved at least 10-fold compared with traditional Au-labeled LFS, guaranteeing accurate screening of trace MOP in the fingerprint sweat. Of great importance, the consumption of valuable antibody can be reduced to just 1/20, which greatly reduces the cost of high-throughput screening. This stepwise Au decoration-assisted double signal amplified LFS holds great potential in the ultrasensitive screening of trace analytes in various fields and further widens the application scope of lateral flow strips.

Superhydrophobic Surface Modification of Polymer Microneedles Enables Fabrication of Multimodal Surface-Enhanced Raman Spectroscopy and Mass Spectrometry Substrates for Synthetic Drug Detection in Blood Plasma

Microneedles are widely used substrates for various chemical and biological sensing applications utilizing surface-enhanced Raman spectroscopy (SERS), which is indeed a highly sensitive and specific analytical approach. This article reports the fabrication of a nanoparticle (NP)-decorated microneedle substrate that is both a SERS substrate and a substrate-supported electrospray ionization (ssESI) mass spectrometry (MS) sample ionization platform. Polymeric ligand-functionalized gold nanorods (Au NRs) are adsorbed onto superhydrophobic surface-modified polydimethylsiloxane (PDMS) microneedles through the control of various interfacial interactions. We show that the chain length of the polymer ligands dictates the NR adsorption process. Importantly, assembling Au NRs onto the micrometer-diameter needle tips allows the formation of highly concentrated electromagnetic hot spots, which provide the SERS enhancement factor as high as 1.0 × 106. The micrometer-sized area of the microneedle top and high electromagnetic field enhancement of our system can be loosely compared with tip-enhanced Raman spectroscopy, where the apex of a plasmonic NP-functionalized sharp probe produces high-intensity plasmonic hot spots. Utilizing our NR-decorated microneedle substrates, the synthetic drugs fentanyl and alprazolam are analyzed with a subpicomolar limit of detection. Further analysis of drug-molecule interactions on the NR surface utilizing the Langmuir adsorption model suggests that the higher polarizability of fentanyl allows for a stronger interaction with hydrophilic polymer layers on the NR surface. We further demonstrate the translational aspect of the microneedle substrate for both SERS- and ssESI-MS-based detection of these two potent drugs in 10 drug-of-abuse (DOA) patient plasma samples with minimal preanalysis sample preparation steps. Chemometric analysis for the SERS-based detection shows a very good classification between fentanyl, alprazolam, or a mixture thereof in our selected 10 samples. Most importantly, ssESI-MS analysis also successfully identifies fentanyl or alprazolam in these same 10 DOA plasma samples. We believe that our multimodal detection approach presented herein is a highly versatile detection technology that can be applicable to the detection of any analyte type without performing any complicated sample preparation.

Evaluation and classification of fentanyl-related compounds using EC-SERS and machine learning

Multiple analytical techniques for the screening of fentanyl-related compounds exist. High discriminatory methods such as GC-MS and LC-MS are expensive, time-consuming, and less amenable to onsite analysis. Raman spectroscopy provides a rapid, inexpensive alternative. Raman variants such as electrochemical surface-enhanced Raman scattering (EC-SERS) can provide signal enhancements with 1010 magnitudes, allowing for the detection of low-concentration analytes, otherwise undetected using conventional Raman. Library search algorithms embedded in instruments utilizing SERS may suffer from accuracy when multicomponent mixtures involving fentanyl derivatives are analyzed. The complexing of machine learning techniques to Raman spectra demonstrates an improvement in the discrimination of drugs even when present in multicomponent mixtures of various ratios. Additionally, these algorithms are capable of identifying spectral features difficult to detect by manual comparisons. Therefore, the goal of this study was to evaluate fentanyl-related compounds and other drugs of abuse using EC-SERS and to process the acquired data using machine learning-convolutional neural networks (CNN). The CNN was created using Keras v 2.4.0 with Tensorflow v 2.9.1 backend. In-house binary mixtures and authentic adjudicated case samples were used to evaluate the created machine-learning models. The overall accuracy of the model was 98.4 ± 0.1% after 10-fold cross-validation. The correct identification for the in-house binary mixtures was 92%, while the authentic case samples were 85%. The high accuracies achieved in this study demonstrate the advantage of using machine learning to process spectral data when screening seized drug materials comprised of multiple components.

Application of Paper-Based Microfluidic Analytical Devices (µPAD) in Forensic and Clinical Toxicology: A Review

The need for providing rapid and, possibly, on-the-spot analytical results in the case of intoxication has prompted researchers to develop rapid, sensitive, and cost-effective methods and analytical devices suitable for use in nonspecialized laboratories and at the point of need (PON). In recent years, the technology of paper-based microfluidic analytical devices (μPADs) has undergone rapid development and now provides a feasible, low-cost alternative to traditional rapid tests for detecting harmful compounds. In fact, µPADs have been developed to detect toxic molecules (arsenic, cyanide, ethanol, and nitrite), drugs, and drugs of abuse (benzodiazepines, cathinones, cocaine, fentanyl, ketamine, MDMA, morphine, synthetic cannabinoids, tetrahydrocannabinol, and xylazine), and also psychoactive substances used for drug-facilitated crimes (flunitrazepam, gamma-hydroxybutyric acid (GHB), ketamine, metamizole, midazolam, and scopolamine). The present report critically evaluates the recent developments in paper-based devices, particularly in detection methods, and how these new analytical tools have been tested in forensic and clinical toxicology, also including future perspectives on their application, such as multisensing paper-based devices, microfluidic paper-based separation, and wearable paper-based sensors.

A Highly Sensitive and Specific Photonic Crystal-Based Opioid Sensor with Rapid Regeneration

Opioid misuse and overdose have caused devastating public health challenges and economic burdens, calling for the need of rapid, accurate sensitive opioid sensors. Here, we report a photonic crystal-based opioid sensor in the total internal reflection configuration, providing label-free, rapid, quantitative measurements through change of the refractive index. The one-dimensional photonic crystal with a defect layer that is immobilized with opioid antibodies acts as a resonator with an open microcavity. The highly accessible structure responds to analytes within a minute after the aqueous opioid solution is introduced, achieving the highest sensitivity of 5688.8 nm/refractive index unit (RIU) at the incident angle of 63.03°. Our sensor shows a limit of detection (LOD) of 7 ng/mL for morphine in phosphate-buffered saline (PBS, pH 7.4) solutions, well below the required clinical detection limit, and an LOD of 6 ng/mL for fentanyl in PBS, close to the clinical requirement. The sensor can selectively detect fentanyl from a mixture of morphine and fentanyl and be regenerated in 2 min with up to 93.66% recovery rate after five cycles. The efficacy of our sensor is further validated in artificial interstitial fluid and human urine samples.

Limit-Defying μ-Total Analysis System: Achieving Part-Per-Quadrillion Sensitivity on a Hierarchical Optofluidic SERS Sensor

Optofluidic sensors have accelerated the growth of smart sensor platforms with improved sensitivity, reliability, and innovation. In this article, we report the integration of a surface-enhanced Raman scattering (SERS) material consisting of silver nanoparticle-decorated diatomaceous earth (AgNPs-DE) with a flow-through microfluidic device, building up a hierarchical structured micro-total analysis system (μ-TAS) capable of achieving part-per-quadrillion (ppq)-level sensitivity. By the synergic integration of millimeter-scale microfluidic devices and porous laboratory filter paper with a micrometer-sized crosslinked cellulosic network that carries SERS-active AgNPs-DE, which possesses submicron to nanometer regimes of photonic crystals and plasmonic nanostructures, we achieved enhanced mass-transfer efficiency and unprecedented detection sensitivity. In our experiment, fentanyl as the testing analyte at different concentrations was measured using a portable Raman spectrometer. The limit of detection (LOD) was estimated to be 10 ppq from a small detection volume of 10 mL with an ultrafast time of sensing (TOS) of 3 min. To attain comparable signals, the traditional soaking method took more than 90 min to detect 10 part-per-trillion fentanyl from a 10 mL sample. Compared with existing SERS sensing results of fentanyl, the limit-defying μ-TAS reduced the LOD-TOS product by almost 4 orders of magnitude, which represents a new stage of ultrafast sensing of extremely low concentration analytes.

Quantitative Raman Cross-Sections and Band Assignments for Fentanyl and Fentanyl Analogs

Raman cross sections and spectra were measured for five synthetic opioid fentanyl analogs: fentanyl citrate, sufentanil citrate, alfentanil HCl, carfentanil oxalate, and remifentanil HCl. The measurements were performed with excitation wavelengths in the visible (532 nm) and near infrared (785 nm). In addition, density functional theory (DFT) calculations were employed to generate simulated spectra of the compounds and aid in identification of the observed spectral modes. These cross-section measurements and calculations were also used to assess results from a series of measurements of fentanyls cut with other powdered materials. These measurements are valuable for assessment of field-deployable Raman chemical sensors for detection of fentanyl and fentanyl analogs, including when mixed with other materials.

Suite of Aptamer-Based Sensors for the Detection of Fentanyl and Its Analogues

Fentanyl and its analogues are potent synthetic opioids that are commonly abused and are currently the number one cause of drug overdose death in the United States. The ability to detect fentanyl with simple, rapid, and low-cost tools is crucial for forensics, medical care, and public safety. Conventional on-site testing options for fentanyl detection─including chemical spot tests, lateral-flow immunoassays, and portable Raman spectrometers─each have their own unique flaws that limit their analytical utility. Here, we have developed a series of new aptamer-based assays and sensors that can detect fentanyl as well as several of its analogues in a reliable, accurate, rapid, and economic manner. These include colorimetric, fluorescent, and electrochemical sensors, which can detect and quantify minute quantities of fentanyl and many of its analogues with no response to other illicit drugs, cutting agents, or adulterants─even in interferent-ridden binary mixtures containing as little as 1% fentanyl. Given the high performance of these novel analytical tools, we foresee the potential for routine use by medical and law enforcement personnel as well as the general public to aid in rapid and accurate fentanyl identification.

Ultra-trace SERS detection of cocaine and heroin using bimetallic gold-silver nanostars (BGNS-Ag)

A rapid, in-field, and reliable method for the detection of illegal drugs of abuse in biological fluids without any sample pretreatment would potentially be helpful for law enforcement, drug control officials, and public healthcare. In this study, we presented a cost-effective and highly reproducible solution-based surface-enhanced Raman scattering (SERS) platform utilizing a portable Raman instrument for fast sensitive SERS detection of illegal drugs, such as cocaine, and heroin in human urine without any sample preprocessing. The SERS platform was constructed for the first time by combining the superior SERS enhancement properties of bimetallic silver-coated gold nanostars (BGNS-Ag) and the advantages of suitable alkaline metal salts such as NaI for SERS signal amplification. The effects of the silver thickness of BGNS-Ag and alkaline salts on the SERS performance were investigated in detail; we observed that the maximum SERS enhancement was obtained for BGNS-Ag with the maximum silver thickness (54 ± 5 nm) in presence of NaI salt. Our SERS platform shows ultra-high sensitivity of cocaine and heroin with a limit of detection (LOD) as low as 10 pg/mL for cocaine and 100 pg/mL for heroin, which was 100 times lower than that of the traditional silver nanoparticle-based illegal drug detection. As a demonstration, the platform was further applied to detect cocaine and heroin spiked in human urine without any sample preprocessing achieving a LOD of 100 pg/mL for cocaine and 1 ng/mL for heroin. Overall, our SERS detection platform shows potential for rapid, onsite, ultra-low-cost portable applications for trace detection of illegal drugs and biomarkers.

Cucurbituril-protected dual-readout gold nanoclusters for sensitive fentanyl detection

A large number of cases showed that fentanyl (FEN) has become the main cause of death from illegal drug overdose owing to its potent effect on respiratory depression, which has emerged as a grave threat to public health and safety. However, traditional analytical methods require cost-prohibitive equipment, complex pretreatment procedures, and technically trained experts, thus highlighting the urgent need to develop a cost-effective, straightforward, and highly sensitive method to detect FEN. This work demonstrated a dual-readout sensor FGGC-AuNCs@Q7 for FEN detection, which is based on the molecular recognition and self-assembly between the macrocycle cucurbit[7]uril (Q7) and FEN, accompanying spontaneous visual Tyndall effect and fluorescence optical responses of the gold nanoclusters within seconds. A detection limit of 1 ng mL^-1 and a linear range of 9 to 148 000 ng mL^-1 were achieved for fluorescence detection on FEN, with favorable selectivity in the presence of other illicit drugs or common interferents. The proposed method has been proved by its satisfactory application for the analysis of human urine.

Strong π-Metal Interaction Enables Liquid Interfacial Nanoarray-Molecule Co-assembly for Raman Sensing of Ultratrace Fentanyl Doped in Heroin, Ketamine, Morphine, and Real Urine

Toward the challenge on reliable determination of trace fentanyl to avoid opioid overdose death in drug crisis, here we realize rapid and direct detection of trace fentanyl in real human urine without pretreatment by a portable surface enhanced Raman spectroscopy (SERS) strategy on liquid/liquid interfacial (LLI) plasmonic arrays. It was observed that fentanyl could interact with the gold nanoparticles (GNPs) surface, facilitate the LLI self-assembly, and consequently amplify the detection sensitivity with a limit of detection (LOD) as low as 1 ng/mL in aqueous solution and 50 ng/mL spiked in urine. Furthermore, we achieve multiplex blind sample recognition and classification of ultratrace fentanyl doped in other illegal drugs, which has extremely low LODs at mass concentrations of 0.02% (2 ng in 10 μg of heroin), 0.02% (2 ng in 10 μg of ketamine), and 0.1% (10 ng in 10 μg of morphine). A logic circuit of the AND gate was constructed for automatic recognition of illegal drugs with or without fentanyl doping. The data-driven analog soft independent modeling model could quickly distinguish fentanyl-doped samples from illegal drugs with 100% specificity. Molecular dynamics (MD) simulation elucidates the underlying molecular mechanism of nanoarray-molecule co-assembly through strong π-metal interactions and the differences in the SERS signal of various drug molecules. It paves a rapid identification, quantification, and classification strategy for trace fentanyl analysis, indicating broad application prospects in response to the opioid epidemic crisis.

Quantitative Label-Free SERS Detection of Trace Fentanyl in Biofluids with a Freestanding Hydrophobic Plasmonic Paper Biosensor

The prevalence of fentanyl abuse raises global public health concerns with an unprecedented surge in overdose deaths. Rapid identification and quantification of fentanyl in biofluids is of paramount importance to combat fentanyl abuse for law enforcement agencies and promptly treat patients for medical professionals. Herein, a freestanding surface-enhanced Raman spectroscopy (SERS) biosensor with excellent condensing enrichment capability, termed FrEnSERS biosensor, is reported for quantitative label-free detection of trace fentanyl in biofluids. This biosensor comprises a reduced graphene oxide membrane decorated with high-density hydrophobic Au nanostars. A combination of the high SERS enhancement and the focusing effect for analyte enrichment of the hydrophobic surface accounts for the remarkable SERS performance of the FrEnSERS biosensor. We demonstrate that the FrEnSERS biosensor achieves the sensitive and quantitative detection of fentanyl in both serum and urine over a wide dynamic range spanning more than 4 orders of magnitude, with a limit of detection of 0.47 ng/mL for serum samples and 0.73 ng/mL for urine samples. Our biosensor is sensitive, cost-effective, and reliable for rapid quantitative analysis of fentanyl in biofluids with great promise for forensic analysis and clinical diagnosis.

MOFs-Modified Electrochemical Sensors and the Application in the Detection of Opioids

Opioids are widely used in clinical practice, but drug overdoses can lead to many adverse reactions, and even endanger life. Therefore, it is essential to implement real-time measurement of drug concentrations to adjust the dosage given during treatment, keeping drug levels within therapeutic levels. Metal-Organic frameworks (MOFs) and their composite materials modified bare electrode electrochemical sensors have the advantages of fast production, low cost, high sensitivity, and low detection limit in the detection of opioids. In this review, MOFs and MOFs composites, electrochemical sensors modified with MOFs for the detection of opioids, as well as the application of microfluidic chips in combination with electrochemical methods are all reviewed, and the potential for the development of microfluidic chips electrochemical methods with MOFs surface modifications for the detection of opioids is also prospected. We hope that this review will provide contributions to the study of electrochemical sensors modified with MOFs for the detection of opioids.

Superhydrophobic 3D-Assembled Metallic Nanoparticles for Trace Chemical Enrichment in SERS Sensing

The performance of surface-enhanced Raman spectroscopy (SERS) is determined by the interaction between highly diluted analytes and boosted localized electromagnetic fields in nanovolumes. Although superhydrophobic surfaces are developed for analyte enrichment, i.e., to concentrate and transfer analytes toward a specific position, it is still challenging to realize reproducible, uniform, and sensitive superhydrophobic SERS substrates over large scales, representing a major barrier for practical sensing applications. To overcome this challenge, a superhydrophobic SERS chip that combines 3D-assembled gold nanoparticles on nanoporous substrates is proposed, for a strong localized field, with superhydrophobic surface treatment for analyte enrichment. Intriguingly, by concentrating droplets in the volume of 40 µL, the sensitivity of 1 nm is demonstrated using 1,2-bis(4-pyridyl)-ethylene molecules. In addition, this unique chip demonstrates a relative standard deviation (RSD) of 2.2% in chip-to-chip reproducibility for detection of fentanyl at 1 µg mL^-1 concentration, revealing its potential for quantitative sensing of chemicals and drugs. Furthermore, the trace analysis of fentanyl and fentanyl-heroin mixture in human saliva is realized after a simple pretreatment process. This superhydrophobic chip paves the way toward on-site and real-time drug sensing to tackle many societal issues like drug abuse and the opioid crisis.

Electrochemical sensing of fentanyl as an anesthesia drug on NiO nanodisks combined with the carbon nanotube-modified electrode

Fentanyl was successfully determined in the current effort based on hexagonal NiO nanodisks (HG-NiO-NDs) fabricated by the hydrothermal protocol. The synergism of HG-NiO-NDs with multiwall carbon nanotubes (MWCNTs), large specific surface area, and active material enabled the electrochemical sensor to show potent electrochemical behavior. Admirable performance was found for the fentanyl measurement by the MWCNT and HG-NiO-ND-modified pencil graphite electrode (MWCNT/HG-NiO-ND/PGE). The correlation of oxidation currents with the pH value, concentration, and sweep rate of supporting electrolytes was determined for the optimization of conditions to detect fentanyl. The surfaces of modified and unmodified electrodes were characterized as well. The diffusion-control processes were confirmed on the basis of anodic peak findings. The results also revealed a two-electron transfer process. The linear range was obtained to be 0.01–800.0 μM for the fentanyl concentrations on the developed electrode, with the sensitivity of 0.1044 μA/mM/cm2. The limit of detection (S/N = 3) was 6.7 nM. The results indicated the ability of the modified electrode to fabricate non-enzymatic fentanyl sensor applications.

Fentanyl Assay Derived from Intermolecular Interaction-Enabled Small Molecule Recognition (iMSR) with Differential Impedance Analysis for Point-of-Care Testing

Rapid and effective differentiation and quantification of a small molecule drug, such as fentanyl, in bodily fluids are major challenges for diagnosis and personal medication. However, the current toxicology methods used to measure drug concentration and metabolites require laboratory-based testing, which is not an efficient or cost-effective way to treat patients in a timely manner. Here, we show an assay for monitoring fentanyl levels by combining the intermolecular interaction-enabled small molecule recognition (iMSR) with differential impedance analysis of conjugated polymers. The differential interactions with the designed anchor interface were transduced through the perturbance of the electric status of the flexible conducting polymer. This assay showed excellent fentanyl selectivity against common interferences, as well as in variable body fluids through either testing strips or skin patches. Directly using the patient blood, the sensor provided 1%-5% of the average deviation compared to the "gold" standard method LC-MS results in the medically relevant fentanyl range of 20-90 nM. The superior sensing properties, in conjunction with mechanical flexibility and compatibility, enabled point-of-care detection and provided a promising avenue for applications beyond the scope of biomarker detection.

Surface Analysis Techniques in Forensic Science: Successes, Challenges, and Opportunities for Operational Deployment

Surface analysis techniques have rapidly evolved in the last decade. Some of these are already routinely used in forensics, such as for the detection of gunshot residue or for glass analysis. Some surface analysis approaches are attractive for their portability to the crime scene. Others can be very helpful in forensic laboratories owing to their high spatial resolution, analyte coverage, speed, and specificity. Despite this, many proposed applications of the techniques have not yet led to operational deployment. Here, we explore the application of these techniques to the most important traces commonly found in forensic casework. We highlight where there is potential to add value and outline the progress that is needed to achieve operational deployment. We consider within the scope of this review surface mass spectrometry, surface spectroscopy, and surface X-ray spectrometry. We show how these tools show great promise for the analysis of fingerprints, hair, drugs, explosives, and microtraces.

Raman inks based on triple-bond-containing polymeric nanoparticles for security

Developing security inks with spectral outputs/multiple colors, which have unique identification characteristics, is of great importance in enhancing the anti-counterfeiting strength of ink anti-counterfeiting technology. Herein, a print-driven triple-bond coding mode is proposed for the first time. Two kinds of triple-bond-containing polymeric nanoparticles (NPs) with Raman shifts at 2227 and 2241 cm^-1 have been designed into printable ink, and the decimal coding output can be easily obtained by reasonably adjusting the proportions of the two polymeric NPs. Single Raman scattering inks can be used as invisible inks to print monochromatic patterns and words that the decoder can read out. According to the two-dimensional pixels of the graphics decoder, invisible colorful graphics can be printed with mixed inks under different polymer proportions. More interestingly, three-dimensional invisible patterns with stronger anti-counterfeiting strength can also be obtained in the double-layer anti-counterfeiting patterns with different proportions of ink by the spatial complementary coding mode. It is predicted that more security inks associated with triple-bond Raman signals will spur the application of the anti-counterfeiting field.

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

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

Research Progress of Raman Spectroscopy and Raman Imaging in Pharmaceutical Analysis

The analytical investigation of the pharmaceutical process monitors the critical process parameters of the drug, beginning from its development until marketing and postmarketing, and appropriate corrective action can be taken to change the pharmaceutical design at any stage of the process. Advanced analytical methods, such as Raman spectroscopy, are particularly suitable for use in the field of drug analysis, especially for qualitative and quantitative work, due to the advantages of simple sample preparation, fast, nondestructive analysis speed, and effective avoidance of moisture interference. Advanced Raman imaging techniques have gradually become a powerful alternative method for monitoring changes in polymorph distribution and active pharmaceutical ingredient distribution in drug processing and pharmacokinetics. Surface-enhanced Raman spectroscopy (SERS) has also solved the inherent insensitivity and fluorescence problems of Raman, which has made good progress in the field of illegal drug analysis. This review summarizes the application of Raman spectroscopy and imaging technology, which are used in the qualitative and quantitative analysis of solid tablets, quality control of the production process, drug crystal analysis, illegal drug analysis, and monitoring of drug dissolution and release in the field of drug analysis in recent years.

Non-contact Detection of Fentanyl by a Field-portable Ion Mobility Spectrometer

Rapid on-site detection of fentanyl is paramount for the safety of law enforcement and other first responders. Due to the opioid epidemic, death by overdose is at an all-time high with fentanyl adulteration as the main assailant. Providing a user-friendly method for the presumptive detection of fentanyl will increase safety for first responders. Ion mobility spectrometry (IMS) provides a quick, affordable, and accurate method for detecting fentanyl. Currently, most methods for detecting fentanyl requires manipulation or handling of the highly potent substance. A recent comparative analysis study on the headspace of fentanyl determined N-phenylpropanamide (NPPA) a target analyte for fentanyl enabling vapor detection. Here, we demonstrate the development of a handheld IMS method for vapor detection of the target analyte for fentanyl. An alarm was programmed into the handheld IMS device for the detection of NPPA. The system was able to accurately detect NPPA in samples of reference-grade fentanyl and diluted reference-grade fentanyl, as well as 3.67 mg of fentanyl from samples confiscated from the U.S. border. Common adulterants and over-the-counter drugs were tested and resulted in a false alarm rate of 0 for substances sampled. The limit of detection was determined to be as low as 5 ng of NPPA. Overall, the development of this user-friendly, non-contact method has considerable promise for near real-time non-contact detection of fentanyl increasing safety of first responders.

Computational analyses of the vibrational spectra of fentanyl, carfentanil and remifentanil

The infrared (IR) spectra of fentanyl, carfentanil and remifentanil, and protonated salts, are computed using quantum chemistry methods. New experimental FTIR spectra are also reported and compared to the calculations. The accuracy of two density functional theory methods, B3LYP and M06-2X, are tested against higher level theories (MP2) and the experimental data. Gas phase IR spectra are calculated for both the neutral and protonated molecules in order to compare with the experimental data measured for various salts of fentanyl and its analogues. Key vibrational modes are selected and studied in detail using a vibrational mode locality calculation. The main contributing atomic movements in these vibrational modes are identified.

An antigen-targeting assay for Lyme disease: Combining aptamers and SERS to detect the OspA protein

Lyme disease is the fastest growing vector-borne disease in the United States. However, current testing modalities are ill suited to detection of Lyme disease, leading to the diagnosis of many cases after treatment is effective. We present an improved, direct method Lyme disease diagnosis, where the Lyme specific biomarker Outer Surface Protein A (OspA) in clinical serum samples is identified using a diagnostic platform combining surface enhanced Raman scattering (SERS) and aptamers. Employing orthogonal projections to latent structures discriminant analysis, the system accurately identified 91% of serum samples from Lyme patients, and 96% of serum samples from symptomatic controls. In addition, the OspA limit-of-detection, determined to be 1 × 10^-4 ng/mL, is greater than four orders of magnitude lower than that found in serum samples from early Lyme disease patients. The application of this platform to detect this difficult-to-diagnose disease suggests its potential for detecting other diseases that present similar difficulties.

Two-Dimensional TiVC Solid-Solution MXene as Surface-Enhanced Raman Scattering Substrate

Two-dimensional (2D) MXenes are attractive candidates as surface-enhanced Raman scattering (SERS) substrates because of their metallic conductivity and abundant surface terminations. Herein, we report the facile synthesis of bimetallic solid-solution TiVC (MXene) and its application in SERS. The few-layered MXene nanosheets with high crystallinity were successfully prepared using a one-step chemical etching method without ultrasonic and organic solvent intercalation steps. SERS activity of the as-prepared MXene was investigated by fabricating free-standing TiVC film as the substrate. A SERS enhancement factor of 10¹² and femtomolar-level detection limit were confirmed using rhodamine 6G as a model dye with 532 nm excitation. The fluorescent signal of the rhodamine 6G dye was effectively quenched, making the SERS spectrum clearly distinguishable. Furthermore, we demonstrate that the TiVC-analyte system with ultrahigh sensitivity is dominated by the chemical mechanism (CM) based on the experimental and simulation results. The abundant density of states near the Fermi level of the TiVC and the strong interaction between the TiVC and analyte promote the intermolecular charge transfer resonance in the TiVC-analyte complex, resulting in significant Raman enhancement. Additionally, several other probe molecules were used for SERS detection to further verify CM-based selectivity enhancement on the TiVC substrates. This work provides guidance for the facile synthesis of 2D MXene and its application in ultrasensitive SERS detection.

Gold-Trisoctahedra-Coated Capillary-Based SERS Platform for Microsampling and Sensitive Detection of Trace Fentanyl

A cost-effective and highly reproducible capillary-based surface-enhanced Raman scattering (SERS) platform for sensitive, portable detection and identification of fentanyl is presented. Through encapsulating gold trisoctahedra (Au TOH) in the capillary tube for the first time, the SERS platform was constructed by combining the superior SERS properties of Au TOH and the advantages of capillaries in SERS signal amplification, facile sample extraction, and portable trace analysis. The effects of the size and density of Au TOH on the SERS performance were investigated by experiments and simulations, which showed that the maximum SERS enhancement was obtained for Au TOH with the size of 75 nm when particle density reached 74.54 counts/μm². The proposed SERS platform possesses good reproducibility with a relative standard deviation (RSD) of less than 5%. As a demonstration, the platform was applied to detect fentanyl spiked in aqueous solution and serum samples with a limit of detection (LOD) as low as 1.86 and 40.63 ng/mL, respectively. We also validated the feasibility of the designed platform for accurate identification of trace fentanyl adulterated in heroin at mass concentration down to 0.1% (10 ng in 10 μg total). Overall, this work advances more explorations on capillary-based SERS platform to benefit portable trace analysis.

Optimization of experimental conditions by surface enhanced Raman Scattering (SERS) spectroscopy with gold nanoparticles suspensions

Surface Enhanced Raman Scattering (SERS) spectroscopy is a rapid and innovative analysis technique involving metallic nanoparticles (NPs). The interaction between NPs and norepinephrine gives an exaltation of the Raman signal under certain experimental conditions. The control of the signal exaltation, crucial for sensitive analyses, remains one of the main limitations of this technique. The aim of this work is to optimize the exaltation conditions for an optimal SERS signal at two concentrations of norepinephrine (NOR) and spherical gold NPs in suspension. This first work will fix the optimal experimental conditions essential for the development of robust discriminant and quantitative analysis of catecholamine. Two complete 3-factors 3-levels experiment designs were performed at 20 µg.mL^-1 and 100 µg.mL^-1 norepinephrine concentrations, each experiment being repeated 3 times. The optimization factors were the process of synthesis (variation of the quantity of gold and citrate used for the three synthesis SA, SB and SC) and HCl (0.3 M, 0.5 M, 0.7 M) as well as the volume ratio of NPs and norepinephrine (0.5, 2, 3.5) for SERS acquisition. Spectral acquisitions were performed with a handheld Raman spectrometer with an excitation source at 785 nm. For each sample, 31 acquisitions were realized during 3 s every 8 s. The optimization parameter was the intensity of the characteristic band of norepinephrine at 1280 cm^-1. A total of 5,042 spectra were acquired and the pre-treatment selected for all spectra was asymmetric least square combined to a smoothing of Savistsky Golay (ALS - SG). The optimal contact time between norepinephrine and NPs depends on the experimental conditions and was determined for each experiment according to the mean intensity between the three replicates. After interpretation of the experimental designs, the optimal conditions retained were the quantity of gold corresponding to SA and the HCl concentration 0.7 M for the two concentrations of norepinephrine. Indeed, the optimal volume ratio depend on the NOR concentration.

Intriguing Cytotoxicity of the Street Dissociative Anesthetic Methoxphenidine: Unexpected Impurities Spotted

The black market for new psychoactive substances has been constantly evolving and the substances that appear on this market cause a considerable number of issues, in extreme cases leading to human deaths. While monitoring the drug black market, we detected a sample of a dissociative anesthetic methoxphenidine, the salt of which contained an unusual anion in the form of bromo- and chloro-zincate complex. Concerning the unknown and potentially hazardous properties of this sample, we performed an in vitro cytotoxicity screening in cell lines of various origins (e.g., kidney, liver, bladder) which was compared with the toxicity results of the methoxphenidine standard prepared for this purpose. The street methoxphenidine sample exhibited markedly higher toxicity than the standard, which was probably caused by the anion impurity. Since it is not usual to analyze anions in salts of novel psychoactive substances, but such samples may be commonly available at the drug black market, we have developed a method for their identification with X-ray powder diffraction (XRPD), which also enabled us to distinguish between different polymorphs/solvates of methoxphenidine that were crystallized in the laboratory. XRPD offers additional data about samples, which may not be discovered by routine techniques, and in some cases, they may help to find out essential information.