Jincymol Kappen

The detection of cancer biomarkers has recently become an established method for the early diagnosis of cancer. The sensitive analysis of specific biomarkers can also be clinically applied for the determination of response to treatment and monitoring of disease progression. Because of the ultra-low concentration of cancer biomarkers in body fluids, diagnostic tools need to be highly sensitive and specific. Conducting polymers (CPs) are particularly known to exhibit numerous features that enable them to serve as excellent materials for the immobilization of biomolecules and the facilitation of electron transfer. Their large surface area, porosity, and the presence of functional groups provide CPs with binding sites suitable for capturing biomarkers, in addition to their sensitive and easy detection. The aim of this review is to present a comprehensive summary of the available electrochemical biosensors based on CPs and their composites for the ultrasensitive detection of selected cancer biomarkers. We have categorized the study based on different types of targeted biomarkers such as DNAs, miRNAs, proteins, enzymes, neurotransmitters and whole cancer cells. The sensitivity of their detection is enhanced by the presence of CPs, providing a limit of detection as low as 0.5 fM (for miRNA) and 10 cells (for the detection of cancer cells). The methods of multiplex biomarker detection and cell capture are indicated as the most promising category, since they furnish more accurate and reliable results. Ultimately, we discuss the available CP-based electrochemical sensors and promising approaches for facilitating cancer diagnosis and treatment.

We report a comprehensive study of the interaction of transition and heavy metal ions with graphene quantum dots-capped silver nanoparticles (AgGQDs) using different spectroscopic and microscopic techniques. High-resolution transmission electron microscopy studies show that the interaction of metal ions with AgGQDs leads to the formation of metal oxides, the formation of zerovalent metals, and the aggregation of Ag nanoparticles (AgNPs). The metal ions may interact with AgGQDs through selective coordination with -OH and -COOH functionalities, adsorption on the graphene moiety, and directly to AgNPs. For instance, the interaction of Cd²⁺ with AgGQDs altered the spherical shape of AgNPs into a chain-like structure. On the contrary, the formation of PbO is observed after the addition of Pb²⁺ to AgGQDs. Interestingly, the interaction of AgGQDs with Hg²⁺ results in the complete dissolution of Ag⁰ from the surface of GQDs and subsequent deposition of Hg⁰ on the graphene moiety of GQDs. Unlike transition metal ions, Cd²⁺, Pb²⁺, and Hg²⁺ can adsorb strongly on the graphene surface at the bridge, hollow, and top sites, respectively. This special interaction of heavy metal ions with the graphene surface would decide the mechanistic pathway in which the reaction proceeds. The transition metal ions Cu²⁺, Zn²⁺, Co³⁺, Mn²⁺, Ni²⁺, and Fe³⁺ induced the aggregation of AgNPs.

The galvanic reaction (GR) between a graphene quantum dot (GQD)-stabilized AgNP (Ag-GQD)-modified glassy carbon (GC) surface and Hg(II) leads to complete dissolution of AgNPs within 15 min and subsequent growth of Hg(0) as a “flower” on the GQD surface. This is unusual because generally the GR of bulk Ag/AgNPs with Hg(II) leads to the formation of a Hg–Ag amalgam/core shell structure. The appearance of peaks at 99.9 and 103.9 eV in X-ray photoelectron spectroscopy confirms Hg(0) on GQDs, whereas the disappearance of a peak at 370 eV indicates complete dissolution of Ag(0). When 200 ppm Hg(II) interacts with Ag-GQDs for 10 min, coalescence of AgNPs takes place along with the formation of Hg(0) petals separately. However, Hg(0) is grown as a flower with 2 μm size, and complete dissolution of AgNPs occurs subsequently after 15 min. The reason for anti-amalgamation is the direct deposition of Hg(0) by the available oxygen functional groups, followed by its strong adsorption on the graphene surface of GQDs. The subsequent growth of Hg(0) as a flower is due to the GR between AgNPs and Hg(II). Interestingly, the Hg flower-GQD-modified GC electrode acts as a good electrocatalyst toward H₂O₂ reduction by decreasing its overpotential by 150 mV in contrast to GC/Ag-GQDs.

Unlike other metals, Hg forms droplets at ambient conditions when a Hg(II) salt interacts with hydroxyl-enriched graphene quantum dots (HEGQDs). The hydroxylation of GQD surface is evident from FT-IR, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The scanning electron microscopy images of Hg(II)-HEGQDs incubated for 0, 1, 24, and 168 h show Hg droplets with the size of 0.1, 0.3, 0.8, and 2 μm, respectively. The XPS studies confirm the presence of Hg(0) and also reveal a noticeable decline in the composition percentage of C–O, whereas a marked increase is observed in the C═O composition percentage. The pathway for the formation of droplets induces immediate reduction of Hg(II) to Hg(0) by both hydroxyl groups and π electron cloud present on the surface of HEGQDs, followed by coalescence. The formed Hg(0) is then strongly adsorbed on the hollow sites of graphene and acts as a nucleation site for the growth of droplets. The kinetics of the reaction obeys LaMer Burst nucleation followed by coalescent growth in addition to autocatalytic reduction and finally follows the Oswald ripening mechanism. The internal pressure of Hg droplets gradually decreases as the radius of the drop increases over the incubation time and liquid–rhombohedral transformation is likely to take place at a radius of 0.8 nm.

Arousing attention has been directed worldwide towards the development of cheap disposable sensors for the detection of clinically important compounds, food contaminants, pesticides and heavy metal ions. One of the most successful disposable sensors to date is the glucometer and its market is likely to reach 15.4 billion dollars by 2026. Keeping the success of the glucometer in mind, scientists continue to search for similar disposable sensors to detect other important biomolecules using nanomaterials. Undoubtedly, one of the most promising two-dimensional nanomaterials is graphene because of its high flexibility, light weight, high surface area, good electrical conductivity and biocompatibility. Graphene and its functionalized forms including graphene oxide and reduced graphene oxide are becoming attractive materials in sensors because of their attractive physical and chemical properties. Although many fantastic reviews and book chapters have been devoted to highlight the developments in the usage of graphene for a variety of sensing applications no attempt has been made to review its application as disposable sensors. Therefore, this chapter aims to cover the synthesis, fabrication and application of graphene as disposable sensors in healthcare.

Paracoccus sp. MKU1, a metabolically versatile bacterium that encompasses diverse metabolic pathways in its genome for the degradation of aromatic compounds, was investigated for catechol bioremediation here for the first time to our knowledge. Paracoccus sp. MKU1 degraded catechol at an optimal pH of 7.5 and a temperature of 37 °C, wherein 100 mg/L catechol was completely mineralized in 96 h but required 192 h for complete mineralization of 500 mg/L catechol. While investigating the molecular mechanisms of its degradation potential, it was unveiled that Paracoccus sp. MKU1 employed both the ortho and meta pathways by inducing the expression of catechol 1,2-dioxygenase (C12O) and catechol 2,3-dioxygenase (C23O), respectively. C23O expression at transcriptional levels was significantly more abundant than C12O, which indicated that catechol degradation was primarily mediated by extradiol cleavage by MKU1. Furthermore, poly(MAA-co-BMA)-GO (PGO) microcomposites containing Paracoccus sp. MKU1 were synthesized, which degraded catechol (100 mg/L) completely within 48 h with excellent recycling performance for three cycles. Thus, [email protected]Paracoccus microcomposites proved to be efficient in catechol degradation at not only faster rates but also with excellent recycling performances than free cells. These findings accomplish that Paracoccus sp. MKU1 could serve as a potential tool for bioremediation of catechol-polluted industrial wastewater and soil.