Plasmonic-based imaging of local square wave voltammetry

Breast cancer detection based on cancer antigen 15-3; emphasis on optical and electrochemical methods: A review

Cancer antigen 15-3 (CA 15-3) is a crucial marker used in the diagnosis and monitoring of breast cancer (BC). The demand for early and precise cancer detection has grown, making the creation of biosensors that are highly sensitive and specific essential. This review paper provides a thorough examination of the progress made in optical and electrochemical biosensors for detecting the cancer biomarker CA 15-3. We focus on explaining their fundamental principles, sensitivity, specificity, and potential for point-of-care applications. The performance attributes of these biosensors are assessed by considering their limits of detection, reaction times, and operational stability, while also making comparisons to conventional methods of CA 15-3 detection. In addition, we explore the incorporation of nanomaterials and innovative transducer components to improve the performance of biosensors. This paper conducts a thorough examination of recent studies to identify the existing obstacles. It also suggests potential areas for future research in this fast progressing field.The paper provides insights into their advancement and utilization to enhance patient outcomes. Both categories of biosensors provide significant promise for the detection of CA 15-3 and offer distinct advantages compared to conventional analytical approaches.

High-resolution impedance mapping using electrically activated quantitative phase imaging

Retrieving electrical impedance maps at the nanoscale rapidly via nondestructive inspection with a high signal-to-noise ratio is an unmet need, likely to impact various applications from biomedicine to energy conversion. In this study, we develop a multimodal functional imaging instrument that is characterized by the dual capability of impedance mapping and phase quantitation, high spatial resolution, and low temporal noise. To achieve this, we advance a quantitative phase imaging system, referred to as epi-magnified image spatial spectrum microscopy combined with electrical actuation, to provide complementary maps of the optical path and electrical impedance. We demonstrate our system with high-resolution maps of optical path differences and electrical impedance variations that can distinguish nanosized, semi-transparent, structured coatings involving two materials with relatively similar electrical properties. We map heterogeneous interfaces corresponding to an indium tin oxide layer exposed by holes with diameters as small as ~550 nm in a titanium (dioxide) over-layer deposited on a glass support. We show that electrical modulation during the phase imaging of a macro-electrode is decisive for retrieving electrical impedance distributions with submicron spatial resolution and beyond the limitations of electrode-based technologies (surface or scanning technologies). The findings, which are substantiated by a theoretical model that fits the experimental data very well enable achieving electro-optical maps with high spatial and temporal resolutions. The virtues and limitations of the novel optoelectrochemical method that provides grounds for a wider range of electrically modulated optical methods for measuring the electric field locally are critically discussed.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Sensitive detection of voltage transients using differential intensity surface plasmon resonance system

This paper describes theoretical and experimental study of the fundamentals of using surface plasmon resonance (SPR) for label-free detection of voltage. Plasmonic voltage sensing relies on the capacitive properties of metal-electrolyte interface that are governed by electrostatic interactions between charge carriers in both phases. Externally-applied voltage leads to changes in the free electron density in the surface of the metal, shifting the SPR position. The study shows the effects of the applied voltage on the shape of the SPR curve. It also provides a comparison between the theoretical and experimental response to the applied voltage. The response is presented in a universal term that can be used to assess the voltage sensitivity of different SPR instruments. Finally, it demonstrates the capacity of the SPR system in resolving dynamic voltage signals; a detection limit of 10mV with a temporal resolution of 5ms is achievable. These findings pave the way for the use of SPR systems in the detection of electrical activity of biological cells.

Plasmonic Imaging of Electrochemical Reactions of Single Nanoparticles

Electrochemical reactions are involved in many natural phenomena, and are responsible for various applications, including energy conversion and storage, material processing and protection, and chemical detection and analysis. An electrochemical reaction is accompanied by electron transfer between a chemical species and an electrode. For this reason, it has been studied by measuring current, charge, or related electrical quantities. This approach has led to the development of various electrochemical methods, which have played an essential role in the understanding and applications of electrochemistry. While powerful, most of the traditional methods lack spatial and temporal resolutions desired for studying heterogeneous electrochemical reactions on electrode surfaces and in nanoscale materials. To overcome the limitations, scanning probe microscopes have been invented to map local electrochemical reactions with nanometer resolution. Examples include the scanning electrochemical microscope and scanning electrochemical cell microscope, which directly image local electrochemical reaction current using a scanning electrode or pipet. The use of a scanning probe in these microscopes provides high spatial resolution, but at the expense of temporal resolution and throughput. This Account discusses an alternative approach to study electrochemical reactions. Instead of measuring electron transfer electrically, it detects the accompanying changes in the reactant and product concentrations on the electrode surface optically via surface plasmon resonance (SPR). SPR is highly surface sensitive, and it provides quantitative information on the surface concentrations of reactants and products vs time and electrode potential, from which local reaction kinetics can be analyzed and quantified. The plasmonic approach allows imaging of local electrochemical reactions with high temporal resolution and sensitivity, making it attractive for studying electrochemical reactions in biological systems and nanoscale materials with high throughput. The plasmonic approach has two imaging modes: electrochemical current imaging and interfacial impedance imaging. The former images local electrochemical current associated with electrochemical reactions (faradic current), and the latter maps local interfacial impedance, including nonfaradic contributions (e.g., double layer charging). The plasmonic imaging technique can perform voltammetry (cyclic or square wave) in an analogous manner to the traditional electrochemical methods. It can also be integrated with bright field, dark field, and fluorescence imaging capabilities in one optical setup to provide additional capabilities. To date the plasmonic imaging technique has found various applications, including mapping of heterogeneous surface reactions, analysis of trace substances, detection of catalytic reactions, and measurement of graphene quantum capacitance. The plasmonic and other emerging optical imaging techniques (e.g., dark field and fluorescence microscopy), together with the scanning probe-based electrochemical imaging and single nanoparticle analysis techniques, provide new capabilities for one to study single nanoparticle electrochemistry with unprecedented spatial and temporal resolutions. In this Account, we focus on imaging of electrochemical reactions at single nanoparticles.

Imaging Local Electric Field Distribution by Plasmonic Impedance Microscopy

We report on imaging of local electric field on an electrode surface with plasmonic electrochemical impedance microscopy (P-EIM). The local electric field is created by putting an electrode inside a micropipet positioned over the electrode and applying a voltage between the two electrodes. We show that the distribution of the surface charge as well as the local electric field at the electrode surface can be imaged with P-EIM. The spatial distribution and the dependence of the local charge density and electric field on the distance between the micropipet and the surface are measured, and the results are compared with the finite element calculations. The work also demonstrates the possibility of integrating plasmonic imaging with scanning ion conductance microscopy (SICM) and other scanning probe microscopies.

Mapping Local Quantum Capacitance and Charged Impurities in Graphene via Plasmonic Impedance Imaging

Local quantum capacitance of graphene is imaged with plasmonics-based electrical impedance microscopy, from which the local density and polarity of charged impurities, electron and hole puddles associated with the charged impurities, and the density of the impurity states are determined.

Electrochemical plasmonic sensing system for highly selective multiplexed detection of biomolecules based on redox nanoswitches

In this paper, we present the development of a nanoswitch-based electrochemical surface plasmon resonance (eSPR) transducer for the multiplexed and selective detection of DNA and other biomolecules directly in complex media. To do so, we designed an experimental set-up for the synchronized measurements of electrochemical and electro-plasmonic responses to the activation of multiple electrochemically labeled structure-switching biosensors. As a proof of principle, we adapted this strategy for the detection of DNA sequences that are diagnostic of two pathogens (drug-resistant tuberculosis and Escherichia coli) by using methylene blue-labeled structure-switching DNA stem-loop. The experimental sensitivity of the switch-based eSPR sensor is estimated at 5 nM and target detection is achieved within minutes. Each sensor is reusable several times with a simple 8M urea washing procedure. We then demonstrated the selectivity and multiplexed ability of these switch-based eSPR by simultaneously detecting two different DNA sequences. We discuss the advantages of the proposed eSPR approach for the development of highly selective sensor devices for the rapid and reliable detection of multiple molecular markers in complex samples.

Detection of charges and molecules with self-assembled nano-oscillators

Detection of a single or small amount of charges and molecules in biologically relevant aqueous solutions is a long-standing goal in analytical science and detection technology. Here we report on self-assembled nano-oscillators for charge and molecular binding detections in aqueous solutions. Each nano-oscillator consists of a nanoparticle linked to a solid surface via a molecular tether. By applying an oscillating electric field normal to the surface, the nanoparticles oscillate, which is detected individually with ∼0.1 nm accuracy by a plasmonic imaging technique. From the oscillation amplitude and phase, the charge of the nanoparticles is determined with a detection limit of ∼0.18 electron charges along with the charge polarity. We further demonstrate the detection of molecular binding with the self-assembled nano-oscillators.

Gold nanoparticle-modified graphite pencil electrode for the high-sensitivity detection of hydrazine

A novel gold nanoparticle-modified graphite pencil electrode (AuNP-GPE) is prepared just by immersing a bare GPE in AuNP solution, followed by heating for 15 min. The bare and modified GPEs are characterized by FE-SEM imaging and cyclic voltammetry. The AuNP-GPEs showed excellent electrocatalytic activities with respect to hydrazine oxidation, with good reproducibility. To reduce the quantification and detection limits, and increase the hydrazine sensitivity, the pH and square wave voltammetry parameters are optimized. A square wave voltammetry study as a function of the hydrazine concentration showed that the AuNP-GPE detector's quantification limit was 100 nmol L(-1) hydrazine, much lower than the value obtained using amperometry (10 µmol L(-1)). The limits of detection (at 3σ) for hydrazine sensing at AuNP-GPEs using square wave voltammetry and amperometry were 42 nmol L(-1) and 3.07 µmol L(-1). Finally, the modified electrode was used to determine the hydrazine concentration in drinking water, and satisfactory results are obtained. This simple, rapid, low-cost method for fabricating a modified electrode is an attractive approach to the development of new sensors.

Imaging the electrocatalytic activity of single nanoparticles

The electrocatalytic properties of nanoparticles depend on their size, shape and composition. These properties are typically probed by measuring the total electrocatalytic reaction current of a large number of nanoparticles, but this approach is time-consuming and can only measure the average catalytic activity of the nanoparticles under study. However, the identification of new catalysts requires the ability to rapidly measure the properties of nanoparticles synthesized under various conditions and, ideally, to measure the electrocatalytic activity of individual nanoparticles. Here, we show that a plasmonic-based electrochemical current-imaging technique can simultaneously image and quantify the electrocatalytic reactions of an array of 1.6 × 10(5) platinum nanoparticles printed on an electrode surface, which could facilitate high-throughput screening of the catalytic activities of nanoparticles. We also show that the approach can be used to image the electrocatalytic reaction current and measure the cyclic voltammograms of single nanoparticles.

Plasmonic-based electrochemical impedance spectroscopy: application to molecular binding

Plasmonic-based electrochemical impedance spectroscopy (P-EIS) is developed to investigate molecular binding on surfaces. Its basic principle relies on the sensitive dependence of surface plasmon resonance (SPR) signal on surface charge density, which is modulated by applying an ac potential to a SPR chip surface. The ac component of the SPR response gives the electrochemical impedance, and the dc component provides the conventional SPR detection. The plasmonic-based impedance measured over a range of frequency is in quantitative agreement with the conventional electrochemical impedance. Compared to the conventional SPR detection, P-EIS is sensitive to molecular binding taking place on the chip surface and less sensitive to bulk refractive index changes or nonspecific binding. Moreover, this new approach allows for simultaneous SPR and surface impedance analysis of molecular binding processes.