Electrochemical determination of riboflavin in pharmaceuticals using unmodified screen printed carbon electrodes.

In this study, we have devised an efficient and rapid approach to detect riboflavin (also known as Vitamin B2 or VB2) utilizing an unaltered screen-printed carbon electrode (SPCE). The unmodified screen-printed electrodes are created within the laboratory, where carbon ink is applied to a ceramic substrate. All experiments pertaining to the investigation of electrochemical behavior and the fine-tuning of crucial experimental parameters were conducted through cyclic voltammetry (CV). For quantitative assessments, square wave voltammetry (SWV) was employed. The findings indicate that unaltered SPCEs exhibit robust current signals during the riboflavin redox reaction. Riboflavin displays a distinct oxidation peak at - 0.136 V (vs. Ag/AgCl, 3.0 M KCl) in a Britton-Robinson buffer solution (BRBS) at pH 2, which was employed for quantification. The electrode demonstrates a broad linear range from 0.05 to 10 µM, boasting a detection limit of 0.03 µM. Repeatability stands at 1.45%, while reproducibility is 6.61%. Testing the influence of common interfering compounds yielded negligible results. The sensor effectively determines riboflavin content in pharmaceutical formulations without any prior treatment. This method presents an economical, modifier-free sensor with exceptional sensitivity and cost-effectiveness, making it suitable for rapid riboflavin quantification.

Hand-Operated, Paper-Based Rotational Vertical-Flow Immunosensor for the Impedimetric Detection of α-Fetoprotein.

This study demonstrates a hand-operated, paper-based rotational vertical-flow immunosensor (rotational VFI) platform requiring fewer pipetting steps, designed for the electrochemical detection of α-fetoprotein with multiple and time-sequenced steps. The platform allows users to perform electrochemical measurements without interference from the convective component of fluid motion, which is unfavorable in most techniques. Users can freely transfer-switch-stop fluid flows by manually rotating the paper disk, evidencing the superior flexibility of this sensor compared to other biosensors. Furthermore, the overall assay duration can be considerably shortened to 9 min. The linear range (LR) is determined to be 0.01-500 ng/mL, with a limit of detection (LOD) of 1.65 pg/mL, and the sensitivity can be significantly enhanced simply by switching off the sample stream to ensure detention at the binding zone (for up to 30 min). This additional step can widen the LR to 0.5 pg/mL, with a LOD of 3.54 fg/mL, which is the lowest detectable level ever reported among paper-based sensors. The advantages of the designed rotational VFI qualify it as a suitable alternative to various biosensors.

A portable selective electrochemical sensor amplified with Fe3O4@Au-cysteamine-thymine acetic acid as conductive mediator for determination of mercuric ion.

Mercury ion (Hg2+) is considered to be one of the most toxic heavy metal ions and can cause adverse effects on kidney function, the central nervous system, and the immune system. Therefore, it is important to develop a fast and simple method for sensitive and selective detection of Hg2+ in the environment. This research proposes a portable electrochemical sensor for rapid and selective detection of Hg2+. The sensor platform is designed based on thymine acetic acid anchored with cysteamine-conjugated core shell Fe3O4@Au nanoparticles (Fe3O4@Au/CA/T-COOH) immobilized on a sensing area of a screen-printed carbon electrode (SPCE) with the aid of an external magnetic field embedded in a homemade electrode holder for ease of handling. In the presence of Hg2+, the immobilized thymine combines specifically with Hg2+ and forms a thymine-Hg2+-thymine mismatch (T-Hg2+-T). The resulting amount of Hg2+ was determined by differential pulse anodic stripping voltammetry (DPASV). Under optimal conditions, the sensor exhibited two wide linearities in a range from 1 to 200 µg L-1 and 200-2200 µg L-1 with the reliability coefficient of determination of 0.997 and 0.999, respectively. The detection limit (LOD) and the quantification limit (LOQ) were also determined to be 0.5 µg L-1 and 1.0 µg L-1, respectively. The sensor was further applied for determination of Hg2+ in water samples, a certified reference material and fish samples. The results were compared with flow injection atomic spectroscopy-inductively coupled plasma-optical emission spectroscopy (FIAS-ICP-OES) systems as a reference method. Results obtained with the proposed sensor were relatively satisfactory, and they showed no significant differences at a 95% confidence level by t-test from the standard method. Therefore, considering its fast and simple advantages, this novel strategy provides a potential platform for construction of a Hg2+ electrochemical sensor.

Wide electrochemical window of screen-printed electrode for determination of rapamycin using ionic liquid/graphene composites.

A disposable screen-printed carbon electrode (SPCE) modified with an ionic liquid/graphene composite (IL/G) exhibits a wider potential window, excellent conductivity, and specific surface area for the improvement in the voltammetric signal of rapamycin detection. The modified composite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). The electrochemical behavior of rapamycin at the modified SPCE was investigated by cyclic and square wave voltammetry in 60:40 EtOH: 0.1 M LiClO4 at pH 5.0. A high reproducible and well-defined peak with a high peak current were obtained for rapamycin detection at a position potential of + 0.98 V versus Ag/AgCl. Under the optimized conditions, the rapamycin concentration in the range 0.1 to 100 µM (R2 = 0.9986) had a good linear relation with the peak current. The detection limit of this method was 0.03 µM (3SD/slope). The proposed device can selectively detect rapamycin in the presence of commonly interfering compounds. Finally, the proposed method was successfully applied to determine rapamycin in urine and blood samples with excellent recoveries. These devices are disposable and cost-effective and might be used as an alternative tool for detecting rapamycin in biological samples and other biological compounds. Graphical abstract Schematic presentation of wide electrochemical window and disposable screen-printed sensor using ionic liquid/graphene composite for the determination of rapamycin. This composite can enhance the oxidation current and expand the potential for rapamycin detection.

Electrochemical immunosensor functionalized with nanobodies for the detection of the toxic microalgae Alexandrium minutum using glassy carbon electrode modified with gold nanoparticles.

In this work an electrochemical immunosensor for the toxic microalgae Alexandrium minutum (A. minutum AL9T) detection is described. A glassy carbon electrode (GCE) was modified by depositing gold nanoparticles followed by L-cysteine for obtaining a self-assembled monolayer. The SpyTagged nanobody C1, specific for the A. minutum toxic strain AL9T, was then covalently immobilized via SpyCatcher on the surface of the modified electrode and used for the selective capture of such microalgae strain. Electrochemical impedance spectroscopy (EIS) was used for the quantification of A. minutum cells present in water samples by measuring the charge-transfer resistance changes of the electrode with a hexacyanoferrate probe. Each electrode modification step was accompanied by cyclic voltammetry (CV) and scanning electron microscopy (SEM). The immunosensor provided highly reproducible data, was simple to fabricate at low cost, exhibited higher sensitivity than previously described alternative diagnostic methods and showed a broad linear range between 103 and 109 cells L-1 with detection limit of 3 × 103 cells L-1 of A. minutum AL9T. The immunosensor was successfully applied to quantify A. minutum AL9T in seawater and brackish water samples proving that it can be used for early detection of harmful microalgae without the necessity of pre-concentration or dialysis steps.

Electrochemical detection of NOx gas based on disposable paper-based analytical device using a copper nanoparticles-modified screen-printed graphene electrode.

A disposable gas-sensing paper-based device (gPAD) was fabricated in origami design which integrates the gas adsorbent and the electrochemical detection zone in a single device. The gPAD for the determination of NOx gas uses a screen-printed graphene electrode modified with copper nanoparticles (CuNP/SPGE) to achieve high sensitivity and selectivity. The gPAD detects both, NO and NO2 (as NOx) with same current responses. The measurement could be performed directly through differential pulse voltammetry (DPV) with a detection limit as low as 0.23 vppm and 0.03 vppm with exposure times of 25 min and 1 h, respectively. The reproducibility in terms of relative standard deviation was less than 5.1% (n = 7 devices) at 25, 75 and 125 vppm NO2 and the life-time of this device was more than 30 days. The gPAD was applied to detect NOx in air and exhaust gases from cars. In comparison with spectrophotometry, there are no significant differences between both methods using a paired t-test of the results on a 95% confidence level. The designed gPAD can provide a new template model for other gas sensors with features of disposability and portability for fieldwork analysis at low cost.

Non-enzymatic electrochemical detection of glucose with a disposable paper-based sensor using a cobalt phthalocyanine-ionic liquid-graphene composite.

We introduce for the first time a paper-based analytical device (PAD) for the non-enzymatic detection of glucose by modifying a screen-printed carbon electrode with cobalt phthalocyanine, graphene and an ionic liquid (CoPc/G/IL/SPCE). The modifying composite was characterized by UV-visible spectroscopy, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The disposable devices show excellent conductivity and fast electron transfer kinetics. The results demonstrated that the modified electrode on PADs had excellent electrocatalytic activity towards the oxidation of glucose with NaOH as supporting electrolyte (0.1M). The oxidation potential of glucose was negatively shifted to 0.64V vs. the screen-printed carbon pseudo-reference electrode. The paper-based sensor comprised a wide linear concentration range for glucose, from 0.01 to 1.3mM and 1.3-5.0mM for low and high concentration of glucose assay, respectively, with a detection limit of 0.67µM (S/N = 3). Additionally, the PADs were applied to quantify glucose in honey, white wine and human serum. The disposable, efficient, sensitive and low-cost non-enzymatic PAD has great potential for the development of point-of-care testing (POCT) devices that can be applied in healthcare monitoring.

Biosensor for the characterisation of hMAO B inhibitors and the quantification of selegiline.

A novel human monoamine oxidase B (hMAO B) based biosensor for inhibitory measurements was developed. It allows both the characterisation of the type of enzyme inhibition and the sensitive and simple determination of inhibitors like selegiline hydrochloride. The sensor consists of a screen printed carbon working electrode modified with 20% manganese dioxide (MnO2) and the enzyme hMAO B, which was immobilised on the electrode via a dialysis membrane (regenerated cellulose, molecular weight cut-off 14000). Inhibition of hMAO B is evaluated by adding different concentrations of the inhibitor selegiline hydrochloride to the enzyme and applying a defined amount of the hMAO B substrate phenylethylamine (PEA). The enzymatically formed H2O2 is amperometrically detected at 0.4V vs. Ag/AgCl in a flow injection analysis (FIA) system. With 100µM PEA the sensor showed a linear correlation between peak height and inhibitor concentration in a range of 0.51-3.25µg/mL selegiline hydrochloride. LOD and LOQ were determined to be 0.15 and 0.51µg/mL, respectively. The sensor showed a repeatability of 3.7% and an intermediate precision of 8.1%. The inhibition-based biosensor was successfully employed to quantify selegiline hydrochloride in pharmaceutical samples. Kinetic studies via Lineweaver-Burk plot and enzyme quantity vs. current plot revealed that the inhibition is irreversible.

Development of Sensitive Analytical Approach for the Quantification of α-Lipoic Acid Using Boron Doped Diamond Electrode.

A boron doped diamond (BDD) electrode was investigated for use as an electrochemical sensor for α-lipoic acid (LA) using amperometric and differential pulse voltammetric detection. LA displays a well expressed oxidation peak at +0.9 V vs. Ag/AgCl in solutions with a pH value of 3. It was found that signals obtained are linearly related to the concentration range from 0.3 to 105 µM with detection limit of 0.088 µM. Interferences by common compounds such as ascorbic acid, uric acid and dopamine were tested and the method was successfully applied to the determination of LA in human body fluids where it gave recoveries in the range from 95 to 97%.

The use of graphene nanoribbons as efficient electrochemical sensing material for nitrite determination.

In this work new designed, highly sensitive electrochemical method is developed for the determination of nitrites in tap water using glassy carbon electrode modified with graphene nanoribbons (GNs/GCE). Graphene nanoribbons (GNs) have been newly synthetized and aligned to the surface of glassy carbon electrode (GCE) and exhibited excellent electrocatalytic activity for nitrite oxidation with a very high peak currents. Studies about electrochemical behavior and optimization of the most important experimental conditions were done using cyclic voltammetry (CV), while quantitative studies were done with amperometric detection. Nitrite provides a well-defined, oxidation peak at +0.9V (vs. Ag/AgCl, 3.0M KCl) in Britton-Robinson buffer solution (BRBS) at pH 3. The influence of most possible interferent ions has been examined and was found to be negligible. Under optimized experimental conditions in BRBS at pH 3 linear calibration curves were obtained in the range from 0.5 to 105µM with the detection limit of 0.22µM. Reproducibility of ten replicate measurements of 1µM of nitrite was estimated to be 1.9%. Proposed method and constructed sensor is successfully applied for the determination of nitrite present in tap water samples without any pretreatment. This developed method represents inexpensive analytical alternative approach compared to other analytical methods.

Manganese dioxide-modified carbon paste electrode for voltammetric determination of riboflavin.

A carbon paste electrode bulk was modified with MnO2 and investigated for use as an electrochemical sensor for riboflavin (vitamin B2) using differential pulse voltammetry (DPV). Riboflavin displays a well expressed oxidation peak at -0.15 V (versus Ag/AgCl) in solutions with a pH value of 2. Effects of pH value, pulse amplitude and pulse time were optimized by employing DPV. The signals obtained are linearly related to the concentrations of riboflavin in the range from 0.02 to 9 µM. Other features include a 15 nM detection limit, and good reproducibility (±3 %) and repeatability (±2 %). Interferences by common compounds were tested, and the method was successfully applied to the determination of riboflavin in pharmaceutical formulations where is gave recoveries in the range from 95 to 97 %. Graphical abstractManganese(IV) oxide was used as a modifier for the carbon paste electrode (MnO2/CPE) for improving its performance toward riboflavin oxidation. Cyclic voltammetry and differential voltammetry were used for characterization and determination of riboflavin, respectively.

Electrochemical sensors for the simultaneous determination of zinc, cadmium and lead using a Nafion/ionic liquid/graphene composite modified screen-printed carbon electrode.

A simple, low cost, and highly sensitive electrochemical sensor, based on a Nafion/ionic liquid/graphene composite modified screen-printed carbon electrode (N/IL/G/SPCE) was developed to determine zinc (Zn(II)), cadmium (Cd(II)), and lead (Pb(II)) simultaneously. This disposable electrode shows excellent conductivity and fast electron transfer kinetics. By in situ plating with a bismuth film (BiF), the developed electrode exhibited well-defined and separate peaks for Zn(II), Cd(II), and Pb(II) by square wave anodic stripping voltammetry (SWASV). Analytical characteristics of the BiF/N/IL/G/SPCE were explored with calibration curves which were found to be linear for Zn(II), Cd(II), and Pb(II) concentrations over the range from 0.1 to 100.0 ng L(-1). With an accumulation period of 120 s detection limits of 0.09 ng mL(-1), 0.06 ng L(-1) and 0.08 ng L(-1) were obtained for Zn(II), Cd(II) and Pb(II), respectively using the BiF/N/IL/G/SPCE sensor, calculated as 3σ value of the blank. In addition, the developed electrode displayed a good repeatability and reproducibility. The interference from other common ions associated with Zn(II), Cd(II) and Pb(II) detection could be effectively avoided. Finally, the proposed analytical procedure was applied to detect the trace metal ions in drinking water samples with satisfactory results which demonstrates the suitability of the BiF/N/IL/G/SPCE to detect heavy metals in water samples and the results agreed well with those obtained by inductively coupled plasma mass spectrometry.