Reliable kinetic modeling of bimolecular reactions from multivariate spectroscopic data often relies on the pseudo-first-order (PFO) approximation. In this approximation, one reactant is assumed to be in constant excess. While this simplification facilitates exponential fitting, its quantitative validity under realistic experimental conditions remains unclear. In this study, we evaluated the accuracy of the PFO approximation compared to the true second-order kinetic model using simulated spectroscopic data generated from a bilinear Beer-Lambert law. The simulations covered factorial combinations of four concentration ratios ([B](0)/[A](0) = 1, 10, 50, and 100), four second-order rate constants (k = 0.04-0.60 M-1 s(-1)), and three noise levels (sigma = 0.0005-0.01 AU). We employed global nonlinear least-squares fitting with a Newton-Gauss/Levenberg-Marquardt algorithm to simultaneously recover the kinetic parameters (k or k(obs)) and pure component spectra. The results showed that the PFO model systematically underestimates the true second-order rate constant when the excess reactant is depleted by more than a few percent. Under near-stoichiometric conditions ([B](0)/[A](0) approximate to 1), recovery errors ranged from 15% to 45% and decreased monotonically with increasing excess. Recovery errors fell below 1 percent only when [B](0)/[A](0) was greater than or equal to 50. Noise had a negligible influence on the mean parameter estimates, but it markedly increased their uncertainties. Therefore, for multivariate spectroscopic kinetic data, the full second-order model should be the default fitting strategy, and the PFO approximation should be reserved for extreme cases of reactant excess where its assumptions are valid.
In this work, chitosan was obtained from shrimp shell waste by acid-alkali extraction. It was then cross-linked with glutaraldehyde to form chitosan hydrogels. The obtained solid particles with average sizes of about 50 mu m (high molecular weight) and 39 mu m (low molecular weight) were prepared and used as adsorbents for Pb (II) and Cd (II) ions from aqueous solutions. The effects of contact time, solution pH, temperature, adsorbent amount, and initial metal ion concentration were investigated. The prepared adsorbents were characterized using SEM, FTIR, and XRD analyses. 200 mg of low molecular weight modified chitosan was able to adsorb 20 mg/L of lead and 2 mg/L of cadmium to the extent of 87.2% and 91.2%, respectively, in 24 h at 30 degrees C and pH=7. The adsorption isotherm of this reaction follows the Langmuir model, and the adsorption process follows a pseudo-second-order kinetic model. The effect of other ions on the adsorption rate was investigated, and it was found that the presence of other metal cations can reduce the adsorption of Pb and Cd by about 15%. The validity and reliability of the method were confirmed through recovery experiments on real contaminated samples, yielding recoveries of 85.8% for Cd and 89.6% for Pb. The reusability of the adsorbent was investigated, and after 6 consecutive adsorption-desorption cycles, the removal efficiency decreased by 10-14%. This study shows that chitosan hydrogels derived from marine biowaste have significant potential as economical, sustainable, and environmentally friendly adsorbents for the treatment of heavy metal-contaminated wastewater.
Exosomes are promising cancer biomarkers but require sensitive, low-cost detection methods. We present a label-free, paper-based electrochemical immunosensor that utilizes a ternary Hemin-mesoporous carbon foam (MCF)-multiwall carbon nanotube (MWCNT) nanocomposite as both an electrode modifier and an internal probe. Hemin-loaded MCF integrated with carboxylated MWCNTs produces a conductive, high-surface-area platform that enables direct signal generation without the need for external labels. The composite was immobilized on a carbon-ink three-electrode paper strip via anti-CD9 antibodies for exosome capture. Electrochemical characterization verified composite formation, antibody attachment, and target binding. Under optimized conditions, the sensor showed a linear response from 5 x 10(2) to 5 x 10(5) exosomes mu l(-1) and a limit of detection of 150 exosomes mu l(-1). DPV currents decreased with increasing exosome load due to the formation of an insulating protein layer, allowing quantification from Delta I. The device exhibited high selectivity, good reproducibility, and stability. Analysis of diluted human serum produced recoveries consistent with ELISA. This low-cost, disposable platform is suitable for rapid exosome analysis and adaptable to multiplexed point-of-care diagnostics.
The development of sensitive and simple methods for determining chemical drugs is of great interest due to their potential side effects. Herein, an electrochemical sensor has been proposed to determine floxuridine (Flx) by utilizing the layered double hydroxide of nickel and cobalt (NiCo-LDH). NiCo-LDH was electrodeposited on the surface of a glassy carbon electrode modified with multi-walled carbon nanotubes (GCE/CNT). LDH synthesis via the electrodeposition technique enjoys advantages such as extreme speed and simplicity. The morphology of the surface of the fabricated sensor was characterized by scanning electron microscopy, and the elemental analysis was performed by energy-dispersive X-ray spectroscopy. To investigate the reactivity of the Flx molecule and identify the sites susceptible to electrochemical oxidation, the structure of Flx was examined by density functional theory (DFT). Comparing the electrochemical behavior of GCE/CNT and GCE/CNT/NiCo-LDH for the determination of Flx in alkaline medium revealed that only NiCo-LDH has an electrocatalytic effect towards the oxidation of Flx. Under optimized conditions, the calibration curve obtained using the differential pulse voltammetry technique was linear from 1.5 x 10(-6) to 2.0 x 10(-4 )M with a detection limit of 5.64 x 10(-8) M. Furthermore, the proposed sensor has good selectivity, excellent precision, and accuracy, and was successfully applied to measure Flx in pharmaceutical and human serum samples with a recovery range of 96.8-104.6%.
The measurement of tramadol as a potent analgesic is essential for the precise control of its dosage to prevent side effects and dependence. This study introduces an innovative electrochemical sensor that utilizes a carbon paste electrode modified with palladium nanoparticlesbonded aluminum oxide nanocomposite (PdNP@Al2O3), resulting in the PdNP@Al2O3 nanocomposite synergistically enhancing the electrocatalytic activity. A multi-technique electrochemical investigation combining cyclic voltammetry (CV), differential pulse voltammetry (DPV), impedance spectroscopy (EIS), chronoamperometry, and chronocoulometry revealed superior redox behavior of tramadol on the PdNP@Al2O3-modified electrode compared to unmodified counterparts. Systematic optimization of operational parameters (scan rate: 5-120 mV s(-1), pH: 3.0-9.0, etc.) yielded a robust sensor with a linear response spanning (0.02-120 mu M) and a record-low detection limit of (1.0 nM). In order to determine the selectivity of the method, the effect of different interference species was studied. Tramadol measurement with the above electrode in real samples, as tablets and blood serum, was examined. Tramadol was also measured in the presence of the narcotic drug morphine to investigate the electrode performance in the presence of other drugs.
