Graphene quantum dots (GQDs), derived from functionalized graphene precursors are graphene sheets a few nanometers in the lateral dimension having a several-layer thickness. They are zero-dimensional materials with quantum confinement and edge site effects. Intense research interest in GQDs is attributed to their unique physicochemical phenomena arising from the sp2-bonded carbon nanocore surrounded with edged plane functional moieties. In this work, GQDs are synthesized by both solvothermal and hydrothermal techniques, with the optimal size of 5 nm determined using high-resolution transmission electron microscopy, with additional UV-Vis absorption and fluorescence spectroscopy, revealing electronic band signatures in the blue-violet region. Their potential in fundamental (direct electron transfer) and applied (enzyme-based glucose biosensor) electrochemistry has been practically realized. Glucose oxidase (GOx) was immobilized on glassy carbon (GC) electrodes modified with GQDs and functionalized graphene (graphene oxide and reduced form). The cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy are used for characterizing the direct electron transfer kinetics and electrocatalytical biosensing. The well-defined quasi-reversible redox peaks were observed under various electrochemical environment and conditions (pH, concentration, scan rate) to determine the diffusion coefficient (D) and first-order electron transfer rate (kET). The cyclic voltammetry curves showed homogeneous ion transport behavior for GQD and other graphene-based samples with D ranging between 8.45 × 10−9 m2 s−1 and 3 × 10−8 m2 s−1 following the order of GO < rGO < GQD < GQD (with FcMeOH as redox probe) < GOx/rGO < GOx/GO < HRP/GQDs < GOx/GQDs. The developed GOx-GQDs biosensor responds efficiently and linearly to the presence of glucose over concentrations ranging between 10 μM and 3 mM with a limit of detection of 1.35 μM and sensitivity of 0.00769 μA μM−1·cm−2 as compared with rGO (0.025 μA μM−1 cm−2, 4.16 μM) and GO (0.064 μA μM−1 cm−2, 4.82 μM) nanosheets. The relatively high performance and stability of GQDs is attributed to a sufficiently large surface-to-volume ratio, excellent biocompatibility, abundant hydrophilic edges, and a partially hydrophobic plane that favors GOx adsorption on the electrode surface and versatile architectures to ensure rapid charge transfer and electron/ion conduction (<10 ms). We also carried out similar studies with other enzymatic protein biomolecules on electrode surfaces prepared from GQD precursors for electrochemical comparison, thus opening up potential sensing applications in medicine as well as bio-nanotechnology.

The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.

Graphene‐mediated surface enhanced Raman scattering is a recent phenomenon that produces clean and reproducible signals from chemical analytes. In this work, we report on the development of graphene‐family nanomaterials (graphene oxide; GO, reduced GO; rGO, and multilayer graphene; MLG) decorated with physisorbed silver (AgNP) and gold (AuNP) nanoparticles and as layered architectures for detection of methylene blue and rhodamine 6G dyes in view of optical and biological significance. The experimental results illustrate four orders of magnitude graphene‐mediated surface enhanced Raman scattering enhancement in the order rGO/AgNP > GO/AgNP > MLG/AgNP for physisorbed and cascade amplified signal on multilayer architectures, larger than those only on graphene and metal nanoparticles, which is achieved at optimal size of Ag (30 nm) and Au (40 nm) on rGO. Moreover, highly‐sensitive graphene‐decorated nanoparticle are capable of molecular detection over a broad concentration range 10 pM–100 μM. The findings are discussed in terms of (a) strong graphene‐metal nanoparticle coupling leading to local interfacial hybridization and polarization, (b) molecular structural symmetry of analytes in relation to nanoparticle‐graphene functionalities, and (c) effective charge transfer and exchange or sharing of charges between analyte and nanoparticles decorated graphene. Optimized metal nanoparticle‐graphene geometries and electronic properties are determined from density functional theory calculations. They identify preferred metal nanoparticle adsorption sites and long‐range electrostatic interactions and determine relative resonant charge transfer population (alternatively, chemical enhancement mechanism) values derived from the Mulliken population thus gaining insights into effective enhancement factors. These findings will help to design advanced SERS platforms for ultrasensitive detection of chemicals and biological molecules useful in bio‐nanotechnology.

Graphene quantum dots (GQDs) - zero-dimensional materials - are sheets of a few nanometers in lateral dimension and exhibit quantum confinement and edge site effects where sp2-bonded carbon nanocore surrounded with edged plane functional moieties is promising as advanced electroactive sensing platforms. In this work, GQDs are synthesized by solvothermal and hydrothermal techniques, with optimal size of 5 nm. Their potential in fundamental (direct electron transfer) and applied (enzymatic glucose biosensor) electrochemistry are demonstrated. Glucose oxidase (GOx) immobilized on glassy carbon (GC) electrodes modified with GQDs are investigated by means of cyclic voltammetry, differential pulse voltammetry, and amperometry. Well-defined quasi-reversible redox peaks observed under various electrochemical parameters helped to determine diffusion coefficient (D) and first-order electron transfer rate (kET). The cyclic voltammetry curves showed homogeneous ion transport for GQD with D ranging between 8.45 × 10−9 m2 s−1 and 3 × 10−8 m2 s−1 following GO < rGO < GQD < GQD (with FcMeOH as redox probe) < GOx/rGO < GOx/GO < HRP/GQDs < GOx/GQDs. The developed GOx-GQDs biosensor responds efficiently and linearly to the presence of glucose over concentrations ranging 10 μM and 3 mM with limit of detection 1.35 μM and sensitivity 0.00769 μA μM−1·cm−2 as compared with rGO (0.025 μA μM−1 cm−2, 4.16 μM) and GO (0.064 μA μM−1 cm−2, 4.82 μM) nanosheets. The high performance and stability of GQDs is attributed to sufficiently large surface-to-volume ratio, excellent biocompatibility, abundant hydrophilic edge site density, and partially hydrophobic planar sites that favors GOx adsorption on the electrode surface and versatile architectures to ensure rapid charge transfer and electron/ion conduction (<10 ms). We also carried out similar studies with other enzymatic protein biomolecules on electrode surfaces prepared from GQD precursors for electrochemical comparison, thus opening up potential sensing applications in medicine as well as bio-nanotechnology.

Noninvasive single cell electrical measurements using carbon nanotubes as electrodes are reported here. The device consists of four nanotubes deposited in the corner of a 2 micron square. Using flow, single cells are places on top of the electrodes. Two of the probes are used to apply voltage pulses to the cell and the other two are used to measure the response as a function of time. As a control, measurements of water, cell medium, cells and biomolecules have been made with metallic plates, defined by 60nm holes in a 75nm insulating film. For proof of principle, yeast cells suspended in HEPES are measured. The results show that the nanotubes allow a contact with the ionic environment 100 times better than the metallic plates. The nanotubes also show a different response when the cell is nearby or touching a cell. Since the nanotubes are 1.2 nm in diameter, comparable in size with membrane proteins, we plan to use the nanotube array to perform some of the functions of patch clamps but with less perturbation to the cells due to the small dimension of carbon nanotubes.