Non-enzymatic Electrochemical Sensor for the Simultaneous Determination of Xanthine, its Methyl Derivatives Theophylline and Caffeine as well as its Metabolite Uric Acid

High-performance colorimetric sensor based on PtRu bimetallic nanozyme for xanthine analysis

The identification and quantification of xanthine are crucial for assessing the freshness and quality of food products, particularly in the seafood industry. Herein, a new approach was developed, involving the in-situ controllable growth of Pt91Ru9 nanoparticles on graphitic carbon nitride to yield Pt91Ru9@C3N4 catalytic materials. By integrating Pt91Ru9@C3N4 with the xanthine/xanthine oxidase (XOD) enzyme catalytic system, a nanozyme-enzyme tandem platform was obtained for the quantification analysis of xanthine. Under the catalytic oxidation of xanthine by XOD in the presence O2, H2O2 was generated. Upon the addition of peroxidase-like activity of Pt91Ru9@C3N4, H2O2 can be decomposed into •OH and 1O2, which can further catalyze the oxidation of TMB to its oxidation product oxTMB with an absorption peak at 652 nm. This smartphone-assisted portable colorimetric sensor for visual monitoring xanthine with a low detection limit of 8.92 nmol L-1, and successfully applied to detect xanthine in grass carp and serum samples.

A Novel Poly(cytosine)-Based Electrochemical Biosensor for Sensitive and Selective Determination of Guanine in Biological Samples

The novel poly(cytosine)-modified glassy carbon electrode-based electrochemical sensor was fabricated potentiodynamically for the detection of Guanine (G) in clinical and biological samples. The surface of the electrode was successfully activated by electropolymerization, and about a 7.5-fold current improvement due to modification was achieved. From the analysis of the dependence of peak current and peak potential on a scan rate, a higher R 2 for the peak current on the square root of scan rate (R 2 = 0.999) than the dependence of peak current on scan rate (R 2 = 0.982) indicated that the oxidation of G at poly(cytosine)/GCE was predominantly diffusion controlled. The oxidative peak response of the electrode revealed a high linear range of G concentration (0.1-200 μM) under optimized conditions. The detection limit and limit of quantification were 6.10 and 20.13 nM, respectively, associated with the %RSD of under 1%. The validation of the developed electrochemical sensor for the determination of G was investigated by analyzing human urine DNA and serum samples with spike recovery results in the range of 98.20-103.70% with the interferent recovery percentage in the range of 97.86-103.10% containing 50-300% of potential interferents. The newly designed sensor demonstrated the highest level of performance for the G detection in real samples.

An Electrochemical Sensor of Theophylline on a Boron-Doped Diamond Electrode Modified with Nickel Nanoparticles

Theophylline is a drug with a narrow therapeutic range. Electrochemical sensors are a potentially effective method for detecting theophylline concentration to prevent toxicity. In this work, a simple modification of a boron-doped diamond electrode using nickel nanoparticles was successfully performed for a theophylline electrochemical sensor. The modified electrode was characterized using a scanning electron microscope and X-ray photoelectron spectroscopy. Square wave voltammetry and cyclic voltammetry methods were used to study the electrochemical behavior of theophylline. The modified nickel nanoparticles on the boron-doped diamond electrode exhibited an electrochemically active surface area of 0.0081 cm2, which is larger than the unmodified boron-doped diamond's area of 0.0011 cm2. This modified electrode demonstrated a low limit of detection of 2.79 µM within the linear concentration range from 30 to 100 µM. Moreover, the modified boron-doped diamond electrode also showed selective properties against D-glucose, ammonium sulfate, and urea. In the real sample analysis using artificial urine, the boron-doped diamond electrode with nickel nanoparticle modifications achieved a %recovery of 105.10%, with a good precision of less than 5%. The results of this work indicate that the developed method using nickel nanoparticles on a boron-doped diamond electrode is promising for the determination of theophylline.

Recent Advances in Electrochemical Sensors for Caffeine Determination

The determination of target analytes at very low concentrations is important for various fields such as the pharmaceutical industry, environmental protection, and the food industry. Caffeine, as a natural alkaloid, is widely consumed in various beverages and medicines. Apart from the beneficial effects for which it is used, caffeine also has negative effects, and for these reasons it is very important to determine its concentration in different mediums. Among numerous analytical techniques, electrochemical methods with appropriate sensors occupy a special place since they are efficient, fast, and entail relatively easy preparation and measurements. Electrochemical sensors based on carbon materials are very common in this type of research because they are cost-effective, have a wide potential range, and possess relative electrochemical inertness and electrocatalytic activity in various redox reactions. Additionally, these types of sensors could be modified to improve their analytical performances. The data available in the literature on the development and modification of electrochemical sensors for the determination of caffeine are summarized and discussed in this review.

Simultaneous detection of purine metabolites by membrane modified electrochemical sensors

Purine metabolites are important for metabolic and cellular processes. Deregulation of purinergic signaling leads to pathological accumulation of purine degradation products in extracellular fluids and indicates various diseases. In clinical diagnosis at early stages of related diseases, accurate detection of Uric acid and Xanthine is of high importance. Electrochemical methods are fast, simple, sensitive, more convenient, and cost-effective compared to other analytical methods used in purine metabolites signaling. Electrochemical sensors are able to detect more compounds simultaneously. Modification of a glassy carbon electrode sensor with external protective membranes was used in this study to avoid unwanted signal interferences from analyte matrices. Polyvinyl alcohol, Chitosan, and Nafion membranes were selected for sensor modification to compare the electro-neutral, positive and negative charged setting of the Xanthine and Uric acid detection. All three membrane modified sensors showed adequate stability in the phosphate buffer solution after 5 min of incubation and are thus suitable for simultaneous detection of purine metabolites. The best results in anodic peak current response values were observed using the Nafion membrane modified glassy carbon electrode sensor. The approach reported here can be useful for the detection of purine metabolites from various matrices at early stages of clinical diagnosis.

Nano optical and electrochemical sensors and biosensors for detection of narrow therapeutic index drugs

For the first time, a comprehensive review is presented on the quantitative determination of narrow therapeutic index drugs (NTIDs) by nano optical and electrochemical sensors and biosensors. NTIDs have a narrow index between their effective doses and those at which they produce adverse toxic effects. Therefore, accurate determination of these drugs is very important for clinicians to provide a clear judgment about drug therapy for patients. Routine analytical techniques have limitations such as being expensive, laborious, and time-consuming, and need a skilled user and therefore  the nano/(bio)sensing technology leads to high interest.

Supercritical fluid extraction-supercritical fluid chromatography of saliva: Single-quadrupole mass spectrometry monitoring of caffeine for gastric emptying studies†

Saliva is an attractive sampling matrix for measuring various endogenous and exogeneous substances but requires sample treatment prior to chromatographic analysis. Exploiting supercritical CO₂ for both extraction and chromatography simplifies sample preparation, reduces organic solvent consumption, and minimizes exposure to potentially infectious samples, but has not yet been applied to oral fluid. Here, we demonstrate the feasibility and benefits of online supercritical fluid extraction coupled to supercritical fluid chromatography and single-quadrupole mass spectrometry for monitoring the model salivary tracer caffeine. A comparison of ¹³ C- and ³² S-labeled internal standards with external standard calibration confirmed the superiority of stable isotope-labeled caffeine over nonanalogous internal standards. As proof of concept, the validated method was applied to saliva from a magnetic resonance imaging study of gastric emptying. After administration of 35 mg caffeine via ice capsule, salivary levels correlated with magnetic resonance imaging data, corroborating caffeine's usefulness as tracer of gastric emptying (R²  = 0.945). In contrast to off-line methods, online quantification required only minute amounts of organic solvents and a single manual operation prior to online bioanalysis of saliva, thus demonstrating the usefulness of CO₂ -based extraction and separation techniques for potentially infective biomatrices.

Cobalt-doped cerium oxide nanocrystals shelled 1D SnO2 structures for highly sensitive and selective xanthine detection in biofluids

In this study, we prepared a three-dimensional self-supported electrocatalyst based on a thin layer of cerium oxide nanocrystals doped with cobalt heteroatoms (CeO₂-Co) and then uniformly shelled over one-dimensional tin oxide (SnO₂) nanorods supported by carbon cloth substrate. The material was used as a binder-free sensor that could nonenzymatically detect xanthine (XA) with an excellent sensitivity of 3.56 μA μM^-1, wide linear range of 25 nM to 55 µM, low detection limit of 58 nM, and good selectivity. A screen-printed electrode based on the material accurately detected XA in food samples as well. The achievements were resulted from synergistic effects coming from the unique core@shell formation and Co-doping strategy, which efficiently modified electronic structure of the material to expose more electroactive site numbers/types and fast charge transfer, thereby producing intrinsic catalytic properties for XA oxidation. These results suggested that the SnO₂@CeO₂-Co is potential for developing efficient sensor to detect XA with good sensitivity and accuracy in food-quality monitoring.

Gold-Platinum Core-Shell Nanoparticles with Thiolated Polyaniline and Multi-Walled Carbon Nanotubes for the Simultaneous Voltammetric Determination of Six Drug Molecules

In this proof-of-concept study, a novel nanocomposite of the thiolated polyaniline (tPANI), multi-walled carbon nanotubes (MWCNTs) and gold–platinum core-shell nanoparticles (Au@Pt) (tPANI-Au@Pt-MWCNT) was synthesized and utilized to modify a glassy carbon electrode (GCE) for simultaneous voltammetric determination of six over-the-counter (OTC) drug molecules: ascorbic acid (AA), levodopa (LD), acetaminophen (AC), diclofenac (DI), acetylsalicylic acid (AS) and caffeine (CA). The nanocomposite (tPANI-Au@Pt-MWCNT) was characterized with transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Using the sensor (GCE-tPANI-Au@Pt-MWCNT) in connection with differential pulse voltammetry (DPV), the calibration plots were determined to be linear up to 570.0, 60.0, 60.0, 115.0, 375.0 and 520.0 µM with limit of detection (LOD) of 1.5, 0.25, 0.15, 0.2, 2.0, and 5.0 µM for AA, LD, AC, DI, AS and CA, respectively. The nanocomposite-modified sensor was successfully used for the determination of these redox-active compounds in commercially available OTC products such as energy drinks, cream and tablets with good recovery yields ranging from 95.48 ± 0.53 to 104.1 ± 1.63%. We envisage that the electrochemical sensor provides a promising platform for future applications towards the detection of redox-active drug molecules in pharmaceutical quality control studies and forensic investigations.

Simultaneous Quantification of Antioxidants Paraxanthine and Caffeine in Human Saliva by Electrochemical Sensing for CYP1A2 Phenotyping

The enzyme CYP1A2 is responsible for the metabolism of numerous antioxidants in the body, including caffeine, which is transformed into paraxanthine, its main primary metabolite. Both molecules are known for their antioxidant and pro-oxidant characteristics, and the paraxanthine-to-caffeine molar ratio is a widely accepted metric for CYP1A2 phenotyping, to optimize dose–response effects in individual patients. We developed a simple, cheap and fast electrochemical based method for the simultaneous quantification of paraxanthine and caffeine in human saliva, by differential pulse voltammetry, using an anodically pretreated glassy carbon electrode. Cyclic voltammetry experiments revealed for the first time that the oxidation of paraxanthine is diffusion controlled with an irreversible peak at ca. +1.24 V (vs. Ag/AgCl) in a 0.1 M H₂SO₄ solution, and that the mechanism occurs via the transfer of two electrons and two protons. The simultaneous quantification of paraxanthine and caffeine was demonstrated in 0.1 M H₂SO₄ and spiked human saliva samples. In the latter case, limits of detection of 2.89 μM for paraxanthine and 5.80 μM for caffeine were obtained, respectively. The sensor is reliable, providing a relative standard deviation within 7% (n = 6). Potential applicability of the sensing platform was demonstrated by running a small scale trial on five healthy volunteers, with simultaneous quantification by differential pulse voltammetry (DPV) of paraxanthine and caffeine in saliva samples collected at 1, 3 and 6 h postdose administration. The results were validated by ultra-high pressure liquid chromatography and shown to have a high correlation factor (r = 0.994).

A highly sensitive gold nanoparticle-based electrochemical aptasensor for theophylline detection

Theophylline is a common bronchodilator for the treatment of diseases like asthma, bronchitis and emphysema. However, it should be strictly used and monitored due to its toxicity when the concentration is above certain levels. In this work, an electrochemical biosensor for theophylline detection is proposed by recognition of RNA aptamer and gold nanoparticle (AuNP)-based amplification technique. First, RNA aptamer is splitted into two single-stranded RNA probes. One is hybridized with DNA tetrahedron and the resulted nanostructure is then immobilized onto a gold electrode; the other is modified on the surface of AuNPs which is also labeled with methylene blue (MB) as electrochemical species. The recognition process between the two RNA probes and theophylline causes the localization of AuNPs and the enrichment of MB on the electrode interface. A significant electrochemical response is thus generated which is related to the concentration of initial theophylline. This proposed aptasensor shows excellent sensitivity and selectivity which could also be applied in quantitatively detection of theophylline in serums samples.