Single-layer graphene as a stable and transparent electrode for nonaqueous radical annihilation electrogenerated chemiluminescence

From theory to practice: understanding the challenges in the implementation of electrogenerated chemiluminescence for analytical applications

Electrogenerated chemiluminescence (ECL) stands out as a remarkable phenomenon of light emission at electrodes initiated by electrogenerated species in solution. Characterized by its exceptional sensitivity and minimal background optical signals, ECL finds applications across diverse domains, including biosensing, imaging, and various analytical applications. This review aims to serve as a comprehensive guide to the utilization of ECL in analytical applications. Beginning with a brief exposition on the theory at the basis of ECL generation, we elucidate the diverse systems employed to initiate ECL. Furthermore, we delineate the principal systems utilized for ECL generation in analytical contexts, elucidating both advantages and challenges inherent to their use. Additionally, we provide an overview of different electrode materials and novel ECL-based protocols tailored for analytical purposes, with a specific emphasis on biosensing applications.

Monitoring SEIRAS on a Graphitic Electrode for Surface-Sensitive Electrochemistry: Real-Time Electrografting

The ubiquity of graphitic materials in electrochemistry makes it highly desirable to probe their interfacial behavior under electrochemical control. Probing the dynamics of molecules at the electrode/electrolyte interface is possible through spectroelectrochemical approaches involving surface-enhanced infrared absorption spectroscopy (SEIRAS). Usually, this technique can only be done on plasmonic metals such as gold or carbon nanoribbons, but a more convenient substrate for carbon electrochemical studies is needed. Here, we expanded the scope of SEIRAS by introducing a robust hybrid graphene-on-gold substrate, where we monitored electrografting processes occurring at the graphene/electrolyte interface. These electrodes consist of graphene deposited onto a roughened gold-sputtered internal reflection element (IRE) for attenuated total reflectance (ATR) SEIRAS. The capabilities of the graphene-gold IRE were demonstrated by successfully monitoring the electrografting of 4-amino-2,2,6,6-tetramethyl-1-piperidine N-oxyl (4-amino-TEMPO) and 4-nitrobenzene diazonium (4-NBD) in real time. These grafts were characterized using cyclic voltammetry and ATR-SEIRAS, clearly showing the 1520 and 1350 cm-1 NO2 stretches for 4-NBD and the 1240 cm-1 C-C, C-C-H, and N-Ȯ stretch for 4-amino-TEMPO. Successful grafts on graphene did not show the SEIRAS effect, while grafting on gold was not stable for TEMPO and had poorer resolution than on graphene-gold for 4-NBD, highlighting the uniqueness of our approach. The graphene-gold IRE is proficient at resolving the spectral responses of redox transformations, unambiguously demonstrating the real-time detection of surface processes on a graphitic electrode. This work provides ample future directions for real-time spectroelectrochemical investigations of carbon electrodes used for sensing, energy storage, electrocatalysis, and environmental applications.

Fully desktop fabricated flexible graphene electrocorticography (ECoG) arrays

Objective:Flexible Electrocorticography (ECoG) electrode arrays that conform to the cortical surface and record surface field potentials from multiple brain regions provide unique insights into how computations occurring in distributed brain regions mediate behavior. Specialized microfabrication methods are required to produce flexible ECoG devices with high-density electrode arrays. However, these fabrication methods are challenging for scientists without access to cleanroom fabrication equipment.Results:Here we present a fully desktop fabricated flexible graphene ECoG array. First, we synthesized a stable, conductive ink via liquid exfoliation of Graphene in Cyrene. Next, we established a stencil-printing process for patterning the graphene ink via laser-cut stencils on flexible polyimide substrates. Benchtop tests indicate that the graphene electrodes have good conductivity of ∼1.1 × 10³S cm^-1, flexibility to maintain their electrical connection under static bending, and electrochemical stability in a 15 d accelerated corrosion test. Chronically implanted graphene ECoG devices remain fully functional for up to 180 d, with averagein vivoimpedances of 24.72 ± 95.23 kΩ at 1 kHz. The ECoG device can measure spontaneous surface field potentials from mice under awake and anesthetized states and sensory stimulus-evoked responses.Significance:The stencil-printing fabrication process can be used to create Graphene ECoG devices with customized electrode layouts within 24 h using commonly available laboratory equipment.

Boosting Electrochemical Activity of Porous Transparent Conductive Oxides Electrodes Prepared by Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is an emerging technique for producing inorganic-organic hybrid materials and templated inorganic nanomaterials. The application space for SIS is expanding rapidly in areas such as lithography, filtration, photovoltaics, antireflection, and triboelectricity, but not in the field of electrochemistry. This study performs SIS for the fabrication of porous, transparent, and electrically conductive films of indium zinc oxide (IZO) to evaluate their potential as an electrode for electrochemistry. The electrochemical activity of IZO-coated electrodes is evaluated when their surfaces are modified with ferrocenecarboxylic acid (FcCOOH), a model redox molecule. Results show a 25-fold enhancement in peak current densities mediated by an Fc/Fc⁺ redox couple for an IZO-coated electrode in comparison with bare electrodes; this is afforded by the porous morphology of the IZO film and the enhanced binding efficiency of FcCOOH on the IZO film. The results confirm the potential of SIS for the preparation of porous transparent conducting oxide electrodes, which will enable the application of SIS-derived materials in various electrochemical fields.

Achieving Fast and Efficient K+ Intercalation on Ultrathin Graphene Electrodes Modified by a Li+ Based Solid-Electrolyte Interphase

Advancing beyond Li-ion batteries requires translating the beneficial characteristics of Li⁺ electrodes to attractive, yet incipient, candidates such as those based on K⁺ intercalation. Here, we use ultrathin few-layer graphene (FLG) electrodes as a model interface to show a dramatic enhancement of K⁺ intercalation performance through a simple conditioning of the solid-electrolyte interphase (SEI) in a Li⁺ containing electrolyte. Unlike the substantial plating occurring in K⁺ containing electrolytes, we found that a Li⁺ based SEI enabled efficient K⁺ intercalation with discrete staging-type phase transitions observed via cyclic voltammetry at scan rates up to 100 mVs^-1 and confirmed as ion-intercalation processes through in situ Raman spectroscopy. The resulting interface yielded fast charge-discharge rates up to ∼360C (1C is fully discharge in 1 h) and remarkable long-term cycling stability at 10C for 1000 cycles. This SEI promoted the transport of K⁺ as verified via mass spectrometric depth profiling. This work introduces a convenient strategy for improving the performance of ion intercalation electrodes toward a practical K-ion battery and FLG electrodes as a powerful analytical platform for evaluating fundamental aspects of ion intercalation.

Single Cell Electrochemiluminescence Imaging: From the Proof-of-Concept to Disposable Device-Based Analysis

We report here the development of coreactant-based electrogenerated chemiluminescence (ECL) as a surface-confined microscopy to image single cells and their membrane proteins. Labeling the entire cell membrane allows one to demonstrate that, by contrast with fluorescence, ECL emission is only detected from fluorophores located in the immediate vicinity of the electrode surface (i.e., 1-2 μm). Then, to present the potential diagnostic applications of our approach, we selected carbon nanotubes (CNT)-based inkjet-printed disposable electrodes for the direct ECL imaging of a labeled plasma receptor overexpressed on tumor cells. The ECL fluorophore was linked to an antibody and enabled to localize the ECL generation on the cancer cell membrane in close proximity to the electrode surface. Such a result is intrinsically associated with the unique ECL mechanism and is rationalized by considering the limited lifetimes of the electrogenerated coreactant radicals. The electrochemical stimulus used for luminescence generation does not suffer from background signals, such as the typical autofluorescence of biological samples. The presented surface-confined ECL microscopy should find promising applications in ultrasensitive single cell imaging assays.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

X-Ray Induced and Thermostimulated Luminescence of New Fluorine Containing Compounds (Potential Luminophores, Scintillators and Dosimeters)

X-ray induced luminescence spectra in optical range of wave-lengths (200-700 nm) and thermoluminescence curves for fluoride, fluorosulphate, fluorophosphate and fluorooxalate compounds of the titanium subgroup elements with alkali metals and ammonium have been obtained. Influence of annealing and repeated X-raying on luminescence (XRL) spectra of a number of compounds has been examined. Alloy additives influence on fluorine compounds XRL spectra has been examined. Most of compounds being under study may be used as X-ray luminophores, scintillators and dosimeters. The highest intensity of luminescence was achieved for such compounds as K₂ZrF₆, Cs₂ZrF₆, Rb₂ZrF₆ (especially for doped K₂ZrF₆), K₂HfF₆ and CsZr₂(PO₄)₃. Radiation drifting to long-wave range for a number of fluorophosphatezirconates in comparison with fluorides may be a positive moment in making luminophores on their base.

Layer Number Dependence of Li(+) Intercalation on Few-Layer Graphene and Electrochemical Imaging of Its Solid-Electrolyte Interphase Evolution

A fundamental question facing electrodes made out of few layers of graphene (FLG) is if they display chemical properties that are different to their bulk graphite counterpart. Here, we show evidence that suggests that lithium ion intercalation on FLG, as measured via stationary voltammetry, shows a strong dependence on the number of layers of graphene that compose the electrode. Despite its extreme thinness and turbostratic structure, Li ion intercalation into FLG still proceeds through a staging process, albeit with different signatures than bulk graphite or multilayer graphene. Single-layer graphene does not show any evidence of ion intercalation, while FLG with four graphene layers displays limited staging peaks, which broaden and increase in number as the layer number increases to six. Despite these mechanistic differences on ion intercalation, the formation of a solid-electrolyte interphase (SEI) was observed on all electrodes. Scanning electrochemical microscopy (SECM) in the feedback mode was used to demonstrate changes in the surface conductivity of FLG during SEI evolution. Observation of ion intercalation on large area FLG was conditioned to the fabrication of "ionic channels" on the electrode. SECM measurements using a recently developed Li-ion sensitive imaging technique evidenced the role of these channels in enabling Li-ion intercalation through localized flux measurements. This work highlights the impact of nanostructure and microstructure on macroscopic electrochemical behavior and provides guidance to the mechanistic control of ion intercalation using graphene, an atomically thin interface where surface and bulk reactivity converge.

Graphene as Transparent Electrodes: Fabrication and New Emerging Applications

Graphene has been regarded as a promising candidate for a new generation of transparent electrodes (TEs) due to its prominent characteristics including high optical transmittance, exceptional electronic transport, outstanding mechanical strength, and environmental stability. Comprehensive and critical insights into the latest advances in graphene-based TEs (GTEs) since, but not limited to 2013, are provided, with an emphasis on fabrication, modification, and versatile applications. Several emerging application areas not previously summarized, including electrochromic devices, supercapacitors, electrochemical and electrochemiluminescent sensors, are discussed in detail. The challenges and prospects in these fields are also addressed.