Low Analyte Concentrations Research Articles

Rapid Determination of the 'Legal Highs' 4-MMC and 4-MEC by Spectroelectrochemistry: Simultaneous Cyclic Voltammetry and In Situ Surface-Enhanced Raman Spectroscopy.

The synthetic cathinones mephedrone (4-MMC) and 4-methylethcathinone (4-MEC) are two designer drugs that represent the rise and fall effect of this drug category within the stimulants market and are still available in several countries around the world. As a result, the qualitative and quantitative determination of ‘legal highs’, and their mixtures, are of great interest. This work explores for the first time the spectroelectrochemical response of these substances by coupling cyclic voltammetry (CV) with Raman spectroscopy in a portable instrument. It was found that the stimulants exhibit a voltammetric response on a gold screen-printed electrode while the surface is simultaneously electro-activated to achieve a periodic surface-enhanced Raman spectroscopy (SERS) substrate with high reproducibility. The proposed method enables a rapid and reliable determination in which both substances can be selectively analyzed through the oxidation waves of the molecules and the characteristic bands of the electrochemical SERS (EC-SERS) spectra. The feasibility and applicability of the method were assessed in simulated seized drug samples and spiked synthetic urine. This time-resolved spectroelectrochemical technique provides a cost-effective and user-friendly tool for onsite screening of synthetic stimulants in matrices with low concentration analytes for forensic applications.

On the Effect of Soft Molecularly Imprinted Nanoparticles Receptors Combined to Nanoplasmonic Probes for Biomedical Applications.

Soft, deformable, molecularly imprinted nanoparticles (nanoMIPs) were combined to nano-plasmonic sensor chips realized on poly (methyl methacrylate) (PMMA) substrates to develop highly sensitive bio/chemical sensors. NanoMIPs (dmean < 50 nm), which are tailor-made nanoreceptors prepared by a template assisted synthesis, were made selective to bind Bovine Serum Albumin (BSA), and were herein used to functionalize gold optical nanostructures placed on a PMMA substrate, this latter acting as a slab waveguide. We compared nanoMIP-functionalized non-optimized gold nanogratings based on periodic nano-stripes to optimized nanogratings with a deposited ultra-thin MIP layer (<100 nm). The sensors performances were tested by the detection of BSA using the same setup, in which both chips were considered as slab waveguides, with the periodic nano-stripes allocated in a longitudinal orientation with respect to the direction of the input light. Result demonstrated the nanoMIP-non optimized nanogratings showed superior performance with respect to the ultra-thin MIP-optimized nanogratings. The peculiar deformable character of the nano-MIPs enabled to significantly enhance the limit of detection (LOD) of the plasmonic bio/sensor, allowing the detection of the low femtomolar concentration of analyte (LOD ∼ 3 fM), thus outpassing of four orders of magnitude the sensitivies achieved so far on optimized nano-patterned plasmonic platforms functionalized with ultra-thin MIP layers. Thus, deformable nanoMIPs onto non-optimized plasmonic probes permit to attain ultralow detections, down to the quasi-single molecule. As a general consideration, the combination of more plasmonic transducers to different kinds of MIP receptors is discussed as a mean to attain the detection range for the selected application field.

Carbon Dioxide Sensing with Langmuir–Blodgett Graphene Films

Graphene has become a material of choice for an increasing number of scientific and industrial applications. It has been used for gas sensing due to its favorable properties, such as a large specific surface area, as well as the sensitivity of its electrical parameters to adsorption processes occurring on its surface. Efforts are ongoing to produce graphene gas sensors by using methods that are compatible with scaling, simple deposition techniques on arbitrary substrates, and ease of use. In this paper, we demonstrate the fabrication of carbon dioxide gas sensors from Langmuir–Blodgett thin films of sulfonated polyaniline-functionalized graphene that was obtained by using electrochemical exfoliation. The sensor was tested within the highly relevant concentration range of 150 to 10,000 ppm and 0% to 100% at room temperature (15 to 35 °C). The results show that the sensor has both high sensitivity to low analyte concentrations and high dynamic range. The sensor response times are approximately 15 s. The fabrication method is simple, scalable, and compatible with arbitrary substrates, which makes it potentially interesting for many practical applications. The sensor is used for real-time carbon dioxide concentration monitoring based on a theoretical model matched to our experimental data. The sensor performance was unchanged over a period of several months.

Rapid screening analysis of antioxidant activities in green tea products using DPPH and FRAP

Free radicals are formed inside and outside the body. Free radicals inside the body can be created in the cell compartments. The reactions are complex through the initiation, propagation, and termination stages. On the other hand, free radicals in the outer body can be formed from pollutions and environmental damage action. Excessive free radicals cause oxidative stress and use destructive cellular to affect health conditions. Antioxidants are known as essential agents in biological systems against diseases triggered by free radicals. One of the many antioxidants found in tea plants (Camellia sinensis) is polyphenols. A colorimetry method was employed to evaluate the dominant antioxidant compound of the total phenolic and flavonoid content in two selected tea products. The total phenolic content was determined by utilizing the Folin-Ciocalteu reagent. In addition, an aluminum colorimetric assay is applied to assess flavonoid content. Rapid screening of antioxidant activity using a spectrophotometric method is employed to determine IC50 values of tea extracts utilizing DPPH and FRAP. The FRAP method showed more sensitivity in determining antioxidant activities with the feasibility of working in low analyte concentrations than the DPPH method. The high total phenolic and flavonoid contents found in this experiment are in line with low IC50 values. It was also found out that brewed green tea has lower total phenol content than green tea bags.

Aluminum Doped ZnO Nanorods for Enhanced Phenylhydrazine Chemical Sensor Applications

The fabrication and characterization of nanorod-shaped Al-doped ZnO nanostructures are described in this paper. The hexagonal wurtzite phase of ZnO was confirmed by XRD investigation, with efficient doping of Aluminum ions in the ZnO crystal. The length and diameter of the high density produced doped ZnO nanorods were shown to be heterogeneous using FESEM analysis. EDS, FTIR, and Raman spectroscopy examinations validated purity, composition, and vibrational characteristics. Even at very low analyte concentrations, the constructed Al-doped ZnO nanorods modified silver electrode (AgE) displayed exceptional electron mediating characteristics towards phenylhydrazine. The sensitivity, linear dynamic concentration range (LDR) and limit of detection (LOD) were found to be 64.77 μAmM−1cm−2, 0.313–2.5 mM, and 0.313 mM, respectively. As a result of the research described, Al-doped ZnO nanorods appear to be promising candidates for building efficient and repeatable electrochemical sensors in the future.

Translatory and rotatory motion of exchange-bias capped Janus particles controlled by dynamic magnetic field landscapes

Magnetic Janus particles (MJPs), fabricated by covering a non-magnetic spherical particle with a hemispherical magnetic in-plane exchange-bias layer system cap, display an onion magnetization state for comparably large diameters of a few microns. In this work, the motion characteristics of these MJPs will be investigated when they are steered by a magnetic field landscape over prototypical parallel-stripe domains, dynamically varied by superposed external magnetic field pulse sequences, in an aqueous medium. We demonstrate, that due to the engineered magnetization state in the hemispherical cap, a comparably fast, directed particle transport and particle rotation can be induced. Additionally, by modifying the frequency of the applied pulse sequence and the strengths of the individual field components, we observe a possible separation between a combined or an individual occurrence of these two types of motion. Our findings bear importance for lab-on-a-chip systems, where particle immobilization on a surface via analyte bridges shall be used for low concentration analyte detection and a particle rotation over a defined position of a substrate may dramatically increase the immobilization (and therefore analyte detection) probability.

Pseudophase microextraction for in-line sample concentration in micellar electrokinetic chromatography

Pseudophase microextraction (PPME) as a simple in-line sample concentration technique in micellar electrokinetic chromatography (MEKC) is presented. In contrast to popular electric-field driven stacking techniques in MEKC such as sweeping, PPME is pressure-driven. The technique afforded up to 403-2968x improvements in peak heights for fenoprop, dichlorprop, 1- and 2-naphthol compared to typical injection. Under the same MEKC conditions, the improvements in PPME were up to 23-59x better compared to sweeping. Briefly in PPME, the entire capillary was loaded (up to 20 capillary volumes) with the sample prepared in a dilute solution of cetyltrimethylammonium bromide ([CTAB] > critical surface aggregation concentration). The CTAB formed aggregates at the inner capillary walls and these aggregates acted as a stationary chromatographic pseudophase. After clean-up via flushing the capillary with purified water, the MEKC background solution (BGS) with sodium dodecyl sulfate was then introduced by pressure from the outlet end to elute the retained analytes. The analytes concentrate at front of the BGS and the front was moved to the inlet end of the capillary prior to MEKC. Optimization strategies and current limitations in PPME-MEKC are described. The linear ranges using a 4 capillary volume sample load obtained for fenoprop, dichlorprop, 1- and 2-naphthol were between 1 and 160 ng/mL (r2s ≥ 0.996), LOQs = 1–2.5 ng/mL and repeatability %RSDs (n = 6) were ≤5% (intra-day) and ≤7% (inter-day) (using low analyte concentrations 1-5x LOQ). PPME-MEKC with simple dilution of fortified real samples (no off-line sample concentration) was also able to detect low levels of dichlorprop (10 ng/mL, limit set in Australia) and 1- and 2−naphthol (7.5–15 ng/mL) in a drinking water and natural water sample, respectively (% recovery = 84–108%). The concept of PPME may find use in other modes of capillary electrophoresis and other nano-microscale separations.

A discussion on the affecting factors of the fitting procedures’ reliability in isothermal titration calorimetry analysis

The standard isothermal titration calorimetry (ITC) curve, characterized as a typical sigmoid is strictly confined by the so-called c value, which is a ratio of titrand concentration to KD. The proper c value with a range from 5 to 500 is commonly recommended as a standard protocol in routine detection process for acquiring the reliable fitting results in 1:1 binding mode. However, if the c value is less than “1” due to the weak binding or low concentration of analyte, fitting precision gets unstable and susceptible to the data noise. Herein, we first got a deep discussion into the reliability of the fitting procedure for 1:1 binding mode by data simulation, then quantized the effect of several affecting factors on the precision of parameters estimation through mathematical analysis. Finally, we proposed the value of 2~4 times KD for final ligand concentration is optimal for the ITC titration in low c system (c < 1). All the theoretical derivations were further verified by a practical experiment of Magnesium-EDTA binding test.

An Immunoenzymatic Method for the Determination of Ochratoxin A in Biological Liquids (Colostrum and Cow's Milk).

Ochratoxins are mycotoxins that have been extensively studied lately due to the multiple toxic effects such as nephrotoxicity, hepatotoxicity, and carcinogenicity. These toxins contaminate plant and animal foods and after ingestion they reach into body fluids. The method of competitive direct enzyme immunoassay, in the solid phase, was validated through the determination of specific parameters (performance, linearity, recovery percentage, limit of detection, limit of quantification). The validated method was used to determine ochratoxin A in colostrum and cow’s milk. The method applied for the determination of ochratoxin A was linear for the concentration range of 0.0–0.5 ng/mL, the value for the regression coefficient (r) was 0.9838. Ochratoxin A was present in 91.67% of the colostrum and in 93.33% of cow’s milk samples. The linearity of the method, demonstrated for very low concentrations of analyte, the detection limit as well as the limit of quantification recommend the method for the determinations of micro-pollutants from foods, including biological fluids.

Dual-ratiometric electrochemical aptasensor enabled by programmable dynamic range: Application for threshold-based detection of aflatoxin B1

Aptamer-based sensor with high-specificity typically has a fixed linear range due to the host-guest binding. To expand the dynamic range for multiple scenarios, we report here a dual-ratiometric electrochemical aptasensor that integrates two aptamer profiles with varied affinity into a single sensing interface for aflatoxin B1 (AFB1) detection. Using functional aptamer as recognition element and generator of signals, we fabricate the dual-ratiometric aptasensor with anthraquinone loaded reduced graphene oxide (AQ-rGO), methylene blue labeled hairpin DNA (MB-DNA), and ferrocene labeled linear aptamer (Fc-apt), using MB and Fc probes to work in tandem while AQ as reference. The current of Fc (IFc) is used to detect the low-concentration analyte, and that of MB (IMB) varies when the concentration of AFB1 reaches a threshold. The threshold switch for IMB could be tuned by engineering the quantity ratio of Fc-apt and MB-DNA (a ratio of 1:1 is used here). The aptasensor can rapidly achieve the positive (+)/negative (-) detection of AFB1 with a threshold concentration of 10 pg mL-1, while the quantitative detection employs the ratio of current of AQ, MB, and Fc (i.e., IAQ/IMB and IAQ/IFc) as yardsticks, offering two linear ranges of 10-106 and 10-2-5 × 104 pg mL-1, respectively. The aptasensor is successfully used to monitor the amount of AFB1 in a 7-days mildew process of peanut. Such a protocol opens a new way for the design of sensors with the programmable linear range in the advanced biological and chemical sensing.

Toward quantitative SERS detection in low analyte concentration by investigating the immersion volume and time of SERS substrate in analyte solution

AbstractThe strength of SERS signal depends on the amount of analyte adsorbed onto the hotspots of a SERS substrate immersed in solution. This adsorption is a dynamic process and can be described by the Langmuir adsorption model, in which the adsorption is influenced by several factors such as the temperature and the immersion time. By varying the immersion time and immersion volume of a gold nanostructure array SERS substrate in malachite green solution, we find that the required immersion time to reach the equilibrium adsorption coverage increases with decreasing analyte concentration and volume. For a 6.5 mm × 6.5 mm SERS substrate immersed in 15 ml of 1.5 ppb malachite solution, it takes more than 7 days of immersion time for it to reach 63% of the equilibrium coverage in ambient environment. Furthermore, at low concentration and immersion volume, the solution concentration decreases during the adsorption process and causes deviation from the prediction of Langmuir isotherm. In this work, we demonstrate that for quantitative SERS measurement in low analyte concentration, it is critical to take the immersion volume and time into consideration and ensure the equilibrium adsorption coverage or SERS intensity is reached for accurate concentration determination.

The new rapid and accurate analytical HPLC‐ECD method for the determination of rosmarinic acid in meat products

A new rapid method has been developed for the determination of low levels of rosmarinic acid extracted from rosemary (Rosmarinus officinalis L.) and has been used as an antioxidant in meat and meat products after cold storage at 4°C. The method is a high performance liquid chromatography using a coulometric electrochemical detector. It provides a significant improvement on the limit of detection, which was 0.33 ppb, while the limit of quantification was 1 ppb of rosmarinic acid. The advantage of the method also lies in the simpler and faster sample preparation, which can quantify a very low concentration of rosmarinic acid (60 ppb), and is more than 40 ppb below the limits of previously existing methods. A coulometric method is well suited for determining low analyte concentrations and is one of the most sensitive analytical approaches available today, in addition to being time efficient and cost effective. PRACTICAL APPLICATION: A new method for determining low concentrations (60 ppb) of rosmarinic acid in meat and meat products is presented. The method is user-friendly, as it does not require complex sample preparation. It is a selective, precise, and accurate method that makes it useful for routine applications in the meat and other food industries.

Sustainable Surface-Enhanced Raman Substrate with Hexagonal Boron Nitride Dielectric Spacer for Preventing Electric Field Cancellation at Au-Au Nanogap.

Nanogaps between Au nanoparticles and Au substrates are the simplest systems that generate extremely high electric fields at hotspots for surface-enhanced Raman spectroscopy (SERS). However, the electric field cancellation at the hotspots in the systems can cause the reduction of Raman signal when two metallic materials are physically contacted due to the low concentration of analytes. Here, we propose an atomically thin hexagonal boron nitride (h-BN) shielding layer for Au substrates, which can be used as an insulating spacer to prevent electrical shorts at nanogaps. Experimental investigation of the SERS effect combined with theoretical studies by finite-difference time-domain simulations demonstrate that the Au NP/h-BN/Au substrate structure has excellent performance in electrical short prevention, thus facilitating ultrasensitive Raman detection. The outstanding chemical and thermal stability of h-BN allow the efficient recycling of the SERS substrate by protecting the Au surface during the removal of Au NPs and molecular analytes by chemical and thermal processes.

A Photoacoustic Contrast Agent for miR-21 via NIR Fluorescent Hybridization Chain Reaction.

Nucleic acids are well-established biomarkers of cancer with immense value in diagnostics and basic research. However, strategies to monitor these species in tissue can be challenging due to the need for amplification of imaging signal from low analyte concentrations with high specificity. Photoacoustic (PA) imaging is gaining traction for molecular imaging of proteins, small biomolecules, and nucleic acids by coupling pulsed near-infrared (NIR) excitation with broadband acoustic detection. This work introduces a PA nucleic acid contrast agent that harnesses NIR fluorophore and quencher-tagged hybridization chain reaction (HCR) for signal amplification. This HCR probe was designed to enable contact quenching between NIR dye-quencher pairs by coercing their direct alignment when miR-21, a microRNA cancer biomarker, is detected. The probe demonstrated a ratiometric PA limit of detection of 148 pM miR-21, sequence specificity against one- and two-base mutations, and selectivity over other microRNAs. It was further tested in live human ovarian cancer (SKOV3) and noncancerous (HEK 293T) cells to exemplify in situ PA activation based on differences in endogenous miR-21 regulation (p = 0.0002). The probe was lastly tested in tissue mimicking phantoms to exemplify sustained contrast in centimeter-range depths and 85.3% photostability after 15 min of laser irradiation. The probe's miR-21-specific activation and its ability to maintain contrast in biologically relevant absorbing and scattering media support its consideration for live-cell PA microscopy and potential cancer diagnostics. Results from this probe also underscore the combined detection power between ratiometric PA signaling and strand amplification for more sensitive DNA-based PA sensors.

A NOVEL ROUTE FOR THE FORMATION OF GAS SENSORS

The rapid development of conductive polymers shows great potential in temperature chemical gas detection as their electrical conductivity is often changed upon spotlight to oxidative or reductive gas molecules at room temperature. However, the relatively low conductivity and high affinity toward volatile organic compounds and water molecules always exhibit low sensitivity, poor stability and gas selectivity, which hinder their practical gas sensor applications. In addition, inorganic sensitive materials show totally different advantages in gas sensors like high sensitivity, fast response to low concentration analytes, high area and versatile surface chemistry, which could harmonize the conducting polymers in terms of the sensing individuality. It seems to be a good option to combine inorganic sensitive materials with polymers for gas detection for the synergistic effects which has attracted extensive interests in gas sensing applications. In this appraisal the recapitulation of recent development in polymer inorganic nanocomposites-based gas sensors. The roles of inorganic nanomaterials in improving the gas sensing performances of conducting polymers are introduced and therefore the progress of conducting polymer inorganic nanocomposites including metal oxides, metal, carbon (carbon nanotube, graphene) and ternary composites are obtainable. Finally, conclusion and perspective within the field of gas sensors incorporating conducting polymer inorganic nanocomposites are summarized. Keywords: Gas sensor, conducting polymer, polymer-inorganic nanocomposites; conducting organic polymers nanostructure, synergistic effect, polypyrrole (PPY), polyaniline (PANI).

Recent developments of X‐ray absorption spectroscopy as analytical tool for biological and biomedical applications

X‐ray absorption spectroscopy (XAS), in its various modalities, has gained exponential attention and applicability in the field of biological and biomedical systems. Particularly in this field, challenges like low concentration of analyte or proneness to radiation damage have certainly settle the basis for further analytical developments, when using X‐ray based methods. Low concentration calls for higher sensitivity—by increasing the detection limits (DL); while susceptibility for radiation damage requires shorter measurement times and/or cryogenic sample environment possibilities. This manuscript reviews the latest analytical possibilities that make XAS more and more adequate to investigate biological or biomedical systems in the last 5 years.

Maneuvering Fluid Motion and Flow-Induced Detection of Toxins by Enzyme Multilayer Films.

In view to develop an autonomous lab-on-a-chip device for detection of toxins without using any spectroscopic or electrochemical equipment, self-powered enzyme micropumps were fabricated via layer-by-layer assembly of enzymes and polyelectrolytes. The thin film-based enzyme micropumps turned on fluid flow in the presence of respective substrates in a concentration-dependent manner, and the rate of the enzymatic reaction was the key for maneuvering the fluid flow. Furthermore, the newly engineered enzyme-based micropumps were able to detect toxic metals and organophosphorus pesticides by modulating the fluid flow speed as the rate of the enzymatic reaction was altered by the presence of inhibitors. Thus, by regulating fluid flow in a micropump, low concentrations of analytes (e.g., target biomarkers and inhibitors) in biological fluids can be quantitatively identified for testing in a resource-constrained environment.

Novel, User-Friendly Experimental and Analysis Strategies for Fast Voltammetry: 1. The Analysis Kid for FSCV.

Fast-scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes measures low concentrations of analytes in biological systems. There are ongoing efforts to simplify FSCV analysis, and several custom platforms are available for filtering and multimodal analysis of FSCV signals, but there is no single, easily accessible platform that has the capacity for all of these features. Here we present The Analysis Kid: currently, the only free, open-source cloud application that does not require a specialized runtime environment and is easily accessible via common browsers. We show that a user-friendly interface can analyze multiplatform file formats to provide multimodal visualization of FSCV color plots with digital background subtraction. We highlight key features that allow interactive calibration and semiautomatic parametric analysis via peak finding algorithms to automatically detect the maximum amplitude, area under the curve, and clearance rate of the signal. Finally, The Analysis Kid enables semiautomatic fitting of data with Michaelis-Menten kinetics with single or dual reuptake models. The Analysis Kid can be freely accessed at http://analysis-kid.hashemilab.com/. The web application code is found, under an MIT license, at https://github.com/sermeor/The-Analysis-Kid.

Sensitivity Analysis of Single- and Bimetallic Surface Plasmon Resonance Biosensors.

Comparative analysis of the sensitivity of two surface plasmon resonance (SPR) biosensors was conducted on a single-metallic Au sensor and bimetallic Ag–Au sensor, using a cathepsin S sensor as an example. Numerically modeled resonance curves of Au and Ag–Au layers, with parameters verified by the results of experimental reflectance measurement of real-life systems, were used for the analysis of these sensors. Mutual relationships were determined between ∂Y/∂n components of sensitivity of the Y signal in the SPR measurement to change the refractive index n of the near-surface sensing layer and ∂n/∂c sensitivity of refractive index n to change the analyte’s concentration, c, for both types of sensors. Obtained results were related to experimentally determined calibration curves of both sensors. A characteristic feature arising from the comparison of calibration curves is the similar level of Au and Ag–Au biosensors’ sensitivity in the linear range, where the signal of the AgAu sensor is at a level several times greater. It was shown that the influence of sensing surface morphology on the ∂n/∂c sensitivity component had to be incorporated to explain the features of calibration curves of sensors. The shape of the sensory surface relief was proposed to increase the sensor sensitivity at low analyte concentrations.

Modelling of plasmonic biosensor temporal response influenced by competitive adsorption and analyte depletion

Highly sensitive detection of biological analytes by plasmonic sensors is based on analyte adsorption, which modifies the effective refractive index (RI) at the medium-sensor interface. The main issue with such sensors is the non-ideal sensor selectivity (manifested in multiple analyte adsorption), while sample depletion (significant at low analyte concentrations) also causes a deviation of the sensor response from the expected time dependence. Mathematical modelling of sensor response, presented in this work, simultaneously tackles these two issues that may cause erroneous interpretation of measurement results. Two mathematical models that address the problem of limited selectivity are utilized to analyse the sensor response: the linear (pseudo-first order) model, commonly used for interpretation of measurements, and the nonlinear (second order) model that takes into account analyte depletion in the sensor chamber. While the nonlinear model has a virtue of generality, the advantages of the linear model are its simplicity and a well-established procedure for the analysis of experimental results. However, the linear model can exhibit large errors in the case of significant analyte depletion. Until now, the influence of analyte depletion on the linear model fidelity has not been studied for multianalyte adsorption. In this paper, we quantify the difference between the RI changes predicted by the two models of two-analyte competitive adsorption, and establish the conditions that justify the use of the linear model. The validity of these conditions is confirmed by numerically solving the exact nonlinear model equations. The conditions enable making an objective decision whether it is safe to use the linear model for detection of ultralow analyte concentrations, or the nonlinear model must be used in order to avoid false-positive/false-negative detection results and analyte quantification errors. The results are applicable in adsorption-based chemical/biological sensing in complex samples, as well as for the new generation single-element multianalyte plasmonic sensors.