Charles C. Chusuei

The relentless pace of industrialization has significantly exacerbated environmental pollution, with heavy-metal ions (HMIs) emerging as some of the most persistent and toxic pollutants in natural ecosystems. Growing concerns over environmental pollution have created a need for advanced sensing technologies that offer superior sensing sensitivity, selectivity, and reliability. This work reports the development of an electrochemical sensor based on a UiO-66-NH₂(Zr) metal–organic framework (MOF)/graphene oxide (GO) nanocomposite for the simultaneous detection of HMIs in aqueous environments. Using one-pot hydrothermal synthesis, MOFs and conductive GO materials were integrated into a single nanostructure via in situ growth of the UiO-66-NH₂(Zr) MOF on the GO matrix, resulting in the formation of a stable MOF/GO nanocomposite with enhanced conductivity and increased number of effective reaction sites. The amino groups (−NH₂) on UiO-66-NH₂(Zr) porous materials serve as adsorption sites to capture HMIs. The morphological, structural, and electrochemical properties of the UiO-66-NH₂(Zr)-GO nanocomposite were examined by using scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Differential pulse anodic stripping voltammetry (DPASV) was subsequently employed for the detection of heavy-metal ions over nanomolar to micromolar concentration ranges using a UiO-66-NH₂(Zr)-GO-modified glassy carbon (GC) electrode. The electrochemical sensor developed in this study was successfully utilized for the selective and concurrent detection of multiple HMIs, namely, copper ion (Cu²⁺), cadmium ion (Cd²⁺), and lead ion (Pb²⁺) in electrolyte solution. The sensor demonstrated achieving high selectivity and sensitivity (1.30 μA μM^–1 for Cu²⁺, 0.50 μA μM^–1 for Cd²⁺, and 12.38 μA μM^–1 for Pb²⁺) with low limit of detection (LOD) (0.59 ng/mL for Cu²⁺, 0.84 ng/mL for Cd²⁺, and 2.9 ng/mL for Pb²⁺), and observed ≥85% reproducibility. The sensor demonstrated excellent long-term stability and operated effectively within a temperature range of 283–313 K, enabling the simultaneous detection of multiple heavy-metal ions from small sample volumes. The developed electrochemical method can equally be employed to detect HMIs at trace (parts-per-billion (ppb)) levels in diverse environmental matrices such as lake, river, tap water, river sediments, and wastewater.

Elevated dopamine (DA) levels in urine denote neuroblastoma, a pediatric cancer. Saccharide-derived carbon dots (CDs) were applied to assay DA detection in simulated urine (SU) while delineating the effects of graphene defect density on electrocatalytic activity. CDs were hydrothermally synthesized to vary graphene defect densities using sucrose, raffinose, and palatinose, depositing them onto glassy carbon electrodes (GCEs). Co₃O₄ nanoparticles (NPs) were encapsulated by the CDs. Cyclic (CV) and linear sweep (LSV) voltammetry measurements were obtained, drop-casting the CDs onto GCEs and measuring DA in a phosphate-buffer solution (pH = 7). DA had an oxidation peak at +0.2 V with SucCDs, with the highest current correlating with the highest defect density. PalCD-Co₃O₄ exhibited the largest signal for DA detection in simulated urine (SU) using the oxidation peak at +0.5 V; the composite had a lower defect density compared to SucCD-Co₃O₄. The Co₃O₄-PalCDs had a DA detection range of 1 to 90 µM with an LOD of 0.88 μM in SU. SEM-EDX analysis of the electrode surface revealed semi-spherical structures with an average particle diameter of 80 ± 19 nm (n = 347) with PalCDs decorating the Co₃O₄ NPs. XRD characterization showed the incorporation of PalCD and Co₃O₄ within the composite. XPS showed electron density donation from the PalCD to Co₃O₄.

Current variance depends upon the differing degrees of graphene defect densities and redox voltage of the analyte for various carbon nanomaterials serving as electrocatalysts.

Neuroblastoma, a pediatric cancer, is characterized by high urinary excretion of dopamine (DA). Silicon nanowires (SiNWs), which are nontoxic and known to resist surface fouling in biological samples, were investigated for practical use as working electrode materials to assay dopamine. Undoped, p-doped, and n-doped SiNWs were deposited onto glassy carbon electrodes (GCEs) and used to measure dopamine in aqueous solution. Within this series, p-doped SiNWs had the highest sensitivity in the series and are the focus of this study. Optimum measurements were obtained at pH 5.0. Cyclic voltammetry (CV) determination of dopamine showed that oxidation at +0.480₁ V versus Ag/AgCl provided the highest sensitivity and also for chronoamperometric (CA) analysis. A calibration curve using chronoamperometry showed a linear (R² = 0.9896) range from 50 to 900 µM. Randles-Sevçik analysis showed that the interaction of dopamine with the electrode surface was diffusion controlled reversibly with coefficients (D) equal to 2.77 × 10⁻⁵ and 2.01 × 10⁻⁵ cm²·s⁻¹ for the oxidation and reduction potentials, respectively. The sensor was demonstrated to be selective in the presence of uric acid, acetaminophen, H₂O₂, folic acid, and glucose.

A Prussian Blue (PB) zinc oxide (ZnO) nanoparticle carboxylic acid-functionalized multiwalled carbon nanotube (PB/ZnO/COOH-MWNT) composite was fabricated for the determination of dopamine (DA). ZnO nanoparticles were tethered to COOH-MWNTs using sonication. Upon attachment of the PB to the ZnO/COOH-MWNTs, which consisted of ZnO nanoparticles 13 nm in diameter, the ZnO coalesced to larger clusters with an average diameter of 573 ± 4 nm. The current response versus various concentrations of DA was measured using chronoamperometry. The optimum conditions for DA measurements were at pH 7.0 using the oxidation current. The sensor has a linear dynamic range from 10 to 900 μM with a limit of detection of 0.37₈ ± 0.01₅ μM, suitable for practical neuroblastoma screening at the lower concentration range and process controls for polydopamine synthesis at the upper concentration range, important for modifying polymeric membranes for water purification.

Carbon nanomaterials (CNMs) have been extensively used as electrochemical sensing composites due to their interesting chemical, electronic, and mechanical properties giving rise to increased performance. Due to these materials’ unknown long-term ecological fate, care must be given to make their use tractable. In this review, the design and use of carbon nanotubes (CNTs), graphene, and carbon dots (CDs) as electrochemical sensing electrocatalysts applied to the working electrode surface are surveyed for various biosensing applications. Graphene and CDs are readily biodegradable as compared to CNTs. Design elements for CNTs that carry over to graphene and CDs include Coulombic attraction of components and using O or N atoms that serve as tethering points for attaching electrocatalytically active nanoparticles (NPs) and/or other additives.

Galactose (Gal), lactose (Lac), and glucose (Glu) derived carbon dots (CDs) were evaluated for their utility as electrochemical sensing composites using acetaminophen (APAP) as a probe molecule. The goal of this work is to ascertain the role of graphene defects on electrochemical activity. Higher sp²‐to‐sp³ hybridized carbon ratios (in parentheses) in the CDs correlated with higher sensitivity in the order according to measured Raman I_G/I_D intensities: GluCDs (6.53)−3 APAP range at pH=7.0 was achieved, suitable for practical APAP toxicity monitoring. Defect density within the GalCDs provided the highest sensitivity.

Carbon dots (C-dots) were facilely fabricated via a hydrothermal method and fully characterized. Our study shows that the as-synthesized C-dots are nontoxic, negatively charged spherical particles (average diameter 4.7 nm) with excellent water dispersion ability. Furthermore, the C-dots have a rich presence of surface functionalities such as hydroxyls and carboxyls as well as amines. The significance of the C-dots as highly efficient photocatalysts for rhodamine B (RhB) and methylene blue (MB) degradation was explored. The C-dots demonstrate excellent photocatalytic activity, achieving 100% of RhB and MB degradation within 170 min. The degradation rate constants for RhB and MB were 1.8 × 10⁻² and 2.4 × 10⁻² min⁻¹, respectively. The photocatalytic degradation performances of the C-dots are comparable to those metal-based photocatalysts and generally better than previously reported C-dots photocatalysts. Collectively considering the excellent photocatalytic activity toward organic dye degradation, as well as the fact that they are facilely synthesized with no need of further doping, compositing, and tedious purification and separation, the C-dots fabricated in this work are demonstrated to be a promising alternative for pollutant degradation and environment protection.

The use of nanomaterial-based products continues to grow with advancing technology. Understanding the potential toxicity of nanoparticles (NPs) is important to ensure that products containing them do not impose harmful effects to human or environmental health. In this study, we evaluated the comparative cytotoxicity between nickel oxide (NiO) and nickel hydroxide (Ni(OH)₂) in human bronchoalveolar carcinoma (A549) and human hepatocellular carcinoma (HepG2) cell lines. Cellular viability studies revealed cell line-specific cytotoxicity in which nickel NPs were toxic to A549 cells but relatively nontoxic to HepG2 cells. Time-, concentration-, and particle-specific cytotoxicity was observed in A549 cells. NP-induced oxidative stress triggered dissipation of mitochondrial membrane potential and induction of caspase-3 enzyme activity. The subsequent apoptotic events led to reduction in cell number. In addition to cell death, suppression of cell proliferation played an essential role in regulating cell number. Collectively, the observed cell viability is a function of cell death and suppression of proliferation. Physical and chemical properties of NPs such as total surface area and metal dissolution are in agreement with the observed differential cytotoxicity. Understanding the properties of NPs is essential in informing the design of safer materials.