Surface potential variations on a silicon nanowire transistor in biomolecular modification and detection

Atomic force microscopy correlates mechanical and electrical properties of HepG2 cells with curcumin concentration

Hepatocellular carcinoma (HCC) is a highly prevalent cancer with a significant impact on human health. Curcumin, a natural compound, induces cytoskeletal changes in liver cancer cells and modifies the distribution of lipids, proteins, and polysaccharides on plasma membranes, affecting their mechanical and electrical properties. In this study, we used nanomechanical indentation techniques and Kelvin probe force microscopy (KPFM) based on atomic force microscopy (AFM) to investigate the changes in surface nanomechanical and electrical properties of nuclear and cytoplasmic regions of HepG2 cells in response to increasing curcumin concentrations. CCK-8 assays and flow cytometry results demonstrated time- and concentration-dependent inhibition of HepG2 cell proliferation by curcumin. Increasing curcumin concentration led to an initial increase and then decrease in the mechanical properties of nuclear and cytoplasmic regions of HepG2 cells, represented by the Young's modulus (E), as observed through nanoindentation. KPFM measurements indicated decreasing trends in both cell surface potential and height. Fluorescence microscopy results indicated a positive correlation between curcumin concentration and phosphatidylserine translocation from the inner to the outer membrane, which influenced the electrical properties of HepG2 cells. This study provides valuable insights into curcumin's mechanisms against cancer cells and aids nanoscale evaluation of therapeutic efficacy and drug screening.

Multifractality in Surface Potential for Cancer Diagnosis

Recent advances in high-resolution biomedical imaging have improved cancer diagnosis, focusing on morphological, electrical, and biochemical properties of cells and tissues, scaling from cell clusters down to the molecular level. Multiscale imaging revealed high complexity that requires advanced data processing methods of multifractal analysis. We performed label-free multiscale imaging of surface potential variations in human ovarian cancer cells using Kelvin probe force microscopy (KPFM). An improvement in the differentiation between nonmalignant and cancerous cells by multifractal analysis using adaptive versus median threshold for image binarization was demonstrated. The results reveal the multifractality of cancer cells as a new biomarker for cancer diagnosis.

High-Performance Bidirectional Chemical Sensor Platform Using Double-Gate Ion-Sensitive Field-Effect Transistor with Microwave-Assisted Ni-Silicide Schottky-Barrier Source/Drain

This study proposes a bidirectional chemical sensor platform using ambipolar double-gate ion-sensitive field-effect transistors (ISFET) with microwave-assisted Ni-silicide Schottky-barrier (SB) source and drain (S/D) on a fully depleted silicon-on-insulator (FDSOI) substrate. The microwave-assisted Ni-silicide SB S/D offer bidirectional turn-on characteristics for both p- and n-type channel operations. The p- and n-type operations are characterized by high noise resistance as well as improved mobility and excellent drift performance, respectively. These features enable sensing regardless of the gate voltage polarity, thus contributing to the use of detection channels based on various target substances, such as cells, antigen-antibodies, DNA, and RNA. Additionally, the capacitive coupling effect existing between the top and bottom gates help achieve self-amplified pH sensitivity exceeding the Nernst limit of 59.14 mV/pH without any additional amplification circuitry. The ambipolar FET sensor performance was evaluated for bidirectional electrical characteristics, pH detection in the single-gate and double-gate modes, and reliability in continuous and repetitive operations. Considering the excellent characteristics confirmed through evaluation, the proposed ambipolar chemical sensor platform is expected to be applicable to various fields including biosensors. And through linkage with subsequent studies, various medical applications and precision detector operations for specific markers will be possible.

Direct observation of surface charge redistribution in active nanoscale conducting channels by Kelvin Probe Force Microscopy

Surface-exposed uniformly doped silicon-on-insulator channels are fabricated to evaluate the accuracy of Kelvin Probe Force Microscopy (KPFM) measured surface potential and reveals the role of surface charge on the exposed channel operated in the ambient environment. First, the quality of the potential profile probed in the vacuum environment is assessed by the consistency of converted resistivity from KPFM result to the resistivity extracted by the other three methods. Second, in contrast to the simulated and vacuum surface potential profile and image, the ambient surface potential is bent excessively at the terminals of the channel. The excessive bending can be explained by the movement of surface charge under the drive of geometry induced strong local electric field from the channel and results in non-uniform distribution. The dynamic movement of surface charges is proved by the observation of time-dependent potential drift in the ambient measurement. The result suggests the surface charge effect should be taken into account of the measurement of the surface potential in the ambient environment and the design of charge sensitive devices whose surfaces are exposed to air or in ambient conditions in their operation.

A supersensitive silicon nanowire array biosensor for quantitating tumor marker ctDNA

Cancer has become one of the major diseases threatening human health and life. Circulating tumor DNA (ctDNA) testing, as a practical liquid biopsy technique, is a promising method for cancer diagnosis, targeted therapy and prognosis. Here, for the first time, a field effect transistor (FET) biosensor based on uniformly sized high-response silicon nanowire (SiNW) array was studied for real-time, label-free, super-sensitive detection of PIK3CA E542K ctDNA. High-response 120-SiNWs array was fabricated on a (111) silicon-on-insulator (SOI) by the complementary metal oxide semiconductor (CMOS)-compatible microfabrication technology. To detecting ctDNA, we modified the DNA probe on the SiNWs array through silanization. The experimental results demonstrated that the as-fabricated biosensor had significant superiority in ctDNA detection, which achieved ultralow detection limit of 10 aM and had a good linearity under the ctDNA concentration range from 0.1 fM to 100 pM. This biosensor can recognize complementary target ctDNA from one/two/full-base mismatched DNA with high selectivity. Furthermore, the fabricated SiNW-array FET biosensor successfully detected target ctDNA in human serum samples, indicating a good potential in clinical applications in the future.

Hybrid Silicon Nanowire Devices and Their Functional Diversity

In the pool of nanostructured materials, silicon nanostructures are known as conventionally used building blocks of commercially available electronic devices. Their application areas span from miniaturized elements of devices and circuits to ultrasensitive biosensors for diagnostics. In this Review, the current trends in the developments of silicon nanowire-based devices are summarized, and their functionalities, novel architectures, and applications are discussed from the point of view of analog electronics, arisen from the ability of (bio)chemical gating of the carrier channel. Hybrid nanowire-based devices are introduced and described as systems decorated by, e.g., organic complexes (biomolecules, polymers, and organic films), aimed to substantially extend their functionality, compared to traditional systems. Their functional diversity is explored considering their architecture as well as areas of their applications, outlining several groups of devices that benefit from the coatings. The first group is the biosensors that are able to represent label-free assays thanks to the attached biological receptors. The second group is represented by devices for optoelectronics that acquire higher optical sensitivity or efficiency due to the specific photosensitive decoration of the nanowires. Finally, the so-called new bioinspired neuromorphic devices are shown, which are aimed to mimic the functions of the biological cells, e.g., neurons and synapses.

A case study of the electrical properties of astrocytes by multimode AFM

Astrocytes play an important role in the physiological functions of the central nervous system. In this study, contact potential differences (CPD) and capacitance gradients of the cell bodies and glial filaments of astrocytes were measured. Charge propagation properties in the astrocyte gap junctions were also studied using multimode Atomic Force Microscopy (AFM) at nanometre resolution. The CPD of the cell bodies and glial filaments were 324.2 ± 138.4 and 119.1 ± 31.7 mV, respectively. The measured capacitance gradients were 1.51 ± 0.31 and 1.98 ± 0.32 zF nm^-1 , respectively. The gap junctions in the astrocytes showed no charge propagation and were not electrically sensitive. This furthers our understanding of astrocytes and other types of neuroglia. LAY DESCRIPTION: Neuroglia cells play important structural and functional roles in central nervous system (CNS). Neuroglia cells exceed the number of neurons by 10∼50 and can be divided into macroglia and microglia. Astrocytes are macroglia and are the largest and most abundant cells in the CNS. Astrocytes lack axons and dendrites and do not propagate action potentials. They have few cytoplasmic organelles, but possess abundant glial filaments, the main components of the cytoskeleton. Glial filaments are composed of the glial fibrillary acidic protein (GFAP). Astrocytes produce intercellular calcium waves in their gap junctions mediated through receptor activator (such as glutamate) to permit signal transduction._ENREF_5 In addition to their role in the support and nutrition of neurons, astrocytes are involved in various types of CNS activity including: (1) cytokine secretion for neuronal survival, growth and differentiation; (2) protection from brain injury; (3) modulation of the blood brain barrier; and (4) neuronal immunity. Bidirectional crosstalk between the astrocytes and neurons exists. Astrocytes can be activated by neurotransmitters released and can themselves release gliotransmitters to act upon neurons. Astrocytes are closely related to various disease states, including epilepsy and Alzheimer's disease. In this study, the electrical properties of astrocytes, including the contact potential difference (CPD) and capacitance gradients of the cell bodies and glial filaments, and charge propagation in the gap junctions were investigated at the nanometer level using quantitative Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM). The CPD of the cell bodies and glial filaments of the astrocytes were 324.2 mV and 119.1 mV, respectively. Capacitance gradients of the cell bodies and glial filaments of the astrocytes were 1.51 zF/nm and 1.98 zF/nm, respectively. Gap junctions in the astrocytes do not perform charge propagation functions and the astrocytes are not electrically sensitive. One should note that these results from KPFM and EFM were measured on dried cell and the situation might be different when studying in operando environment, still these findings aid our understanding of the electrical properties and functions of astrocytes, and further our knowledge of the electrical properties of the CNS.

Direct study of the electrical properties of PC12 cells and hippocampal neurons by EFM and KPFM

Electrical related properties play important roles in biological structures and functions. Herein, the capacitance gradient and local contact potential difference (CPD) of cell bodies and processes of PC12 cells (representative cells of the sympathetic nervous system), hippocampal neurons (representative cells of the central nervous system) and spines were investigated by Electrostatic Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KPFM) at high lateral spatial resolution directly. The results demonstrate that the capacitance gradients of cell bodies, processes and spines of PC12 cells and hippocampal neurons are very close (in the range of 19-23 zF nm-1) and fit well with the theoretical calculation results (21.7 zF nm-1). This indicates that the differences of nerve signal activities and functions of the sympathetic and central nervous systems are not related to the electric polarization properties. The CPD of cell bodies and processes of PC12 cells is smaller than that of hippocampal neurons. The CPD of spines is much more negative than that of the cell bodies and processes. These results reveal that the surface potential is closely related to the neural signal transduction functions, and spines play vital roles in neural signal transmission. This work indicates the similarity (capacitance gradient) and differences (surface potential) of the electrical properties between the sympathetic and central nervous systems for the first time. The methods and results of this work are useful in the further study of the electrical properties in cellular activities and physiological processes.

Beyond the Debye length in high ionic strength solution: direct protein detection with field-effect transistors (FETs) in human serum

In this study, a new type of field-effect transistor (FET)-based biosensor is demonstrated to be able to overcome the problem of severe charge-screening effect caused by high ionic strength in solution and detect proteins in physiological environment. Antibody or aptamer-immobilized AlGaN/GaN high electron mobility transistors (HEMTs) are used to directly detect proteins, including HIV-1 RT, CEA, NT-proBNP and CRP, in 1X PBS (with 1%BSA) or human sera. The samples do not need any dilution or washing process to reduce the ionic strength. The sensor shows high sensitivity and the detection takes only 5 minutes. The designs of the sensor, the methodology of the measurement, and the working mechanism of the sensor are discussed and investigated. A theoretical model is proposed based on the finding of the experiments. This sensor is promising for point-of-care, home healthcare, and mobile diagnostic device.

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.

A hemicellulose-bound form of silicon inhibits cadmium ion uptake in rice (Oryza sativa) cells

Silicon (Si) alleviates cadmium (Cd) toxicity in rice (Oryza sativa). However, the chemical mechanisms at the single-cell level are poorly understood. Here, a suspension of rice cells exposed to Cd and/or Si treatments was investigated using a combination of plant cell nutritional, molecular biological, and physical techniques including in situ noninvasive microtest technology (NMT), polymerase chain reaction (PCR), inductively coupled plasma mass spectroscopy (ICP-MS), and atomic force microscopy (AFM) in Kelvin probe mode (KPFM). We found that Si-accumulating cells had a significantly reduced net Cd(2+) influx, compared with that in Si-limited cells. PCR analyses of the expression levels of Cd and Si transporters in rice cells showed that, when the Si concentration in the medium was increased, expression of the Si transporter gene Low silicon rice 1 (Lsi1) was up-regulated, whereas expression of the gene encoding the transporter involved in the transport of Cd, Natural resistance-associated macrophage protein 5 (Nramp5), was down-regulated. ICP-MS results revealed that 64% of the total Si in the cell walls was bound to hemicellulose constituents following the fractionation of the cell walls, and consequently inhibited Cd uptake. Furthermore, AFM in KPFM demonstrated that the heterogeneity of the wall surface potential was higher in cells cultured in the presence of Si than in those cultured in its absence, and was homogenized after the addition of Cd. These results suggest that a hemicellulose-bound form of Si with net negative charges is responsible for inhibition of Cd uptake in rice cells by a mechanism of [Si-hemicellulose matrix]Cd complexation and subsequent co-deposition.

Advances in nanowire transistors for biological analysis and cellular investigation

Electrical biosensors based on silicon nanowire field-effect transistors (SiNW-FETs) have attracted enormous interest in the biosensing field. SiNW-FETs have proven to be significant and efficient in detecting diverse biomolecular species with the advantages of high probing sensitivity, target selectivity, real-time recording and label-free detection. In recent years, significant advances in biosensors have been achieved, particularly for cellular investigation and biomedical diagnosis. In this critical review, we will report on the latest developments in biosensing with SiNW-FETs and discuss recent advancements in the innovative designs of SiNW-FET devices. This critical review introduces the basic instrumental setup and working principle of SiNW-FETs. Technical approaches that attempted to enhance the detection sensitivity and target selectivity of SiNW-FET sensors are discussed. In terms of applications, we review the recent achievements with SiNW-FET biosensors for the investigations of protein-protein interaction, DNA/RNA/PNA hybridization, virus detection, cellular recording, biological kinetics, and clinical diagnosis. In addition, the novel architecture designs of the SiNW-FET devices are highlighted in studies of live neuron cells, electrophysiological measurements and other signal transduction pathways. Despite these remarkable achievements, certain improvements remain necessary in the device performance and clinical applications of FET-based biosensors; thus, several prospects about the future development of nanowire transistor-based instruments for biosensing employments are discussed at the end of this review.

Nanowire transistor-based ultrasensitive virus detection with reversible surface functionalization

We have applied a reusable silicon nanowire field-effect transistor (SiNW-FET) as a biosensor to conduct ultrasensitive detection of H5N2 avian influenza virus (AIV) in very dilute solution. The reversible surface functionalization of SiNW-FET was made possible using a disulfide linker. In the surface functionalization, 3-mercaptopropyltrimethoxysilane (MPTMS) was first modified on the SiNW-FET (referred to as MPTMS/SiNW-FET), with subsequent dithiothreitol washing to reduce any possible disulfide bonding between the thiol groups of MPTMS. Subsequently, receptor molecules could be immobilized on the MPTMS/SiNW-FET by the formation of a disulfide bond. The success of the reversible surface functionalization was verified with fluorescence examination and electrical measurements. A surface topograph of the SiNW-FET biosensor modified with a monoclonal antibody against H5N2 virus (referred to as mAb(H5)/SiNW-FET) after detecting approximately 10(-17) M H5N2 AIVs was scanned by atomic force microscopy to demonstrate that the SiNW-FET is capable of detecting very few H5N2 AIV particles.

Changes in plasma membrane surface potential of PC12 cells as measured by Kelvin probe force microscopy

The plasma membrane of a cell not only works as a physical barrier but also mediates the signal relay between the extracellular milieu and the cell interior. Various stimulants may cause the redistribution of molecules, like lipids, proteins, and polysaccharides, on the plasma membrane and change the surface potential (Φ_s). In this study, the Φ_ss of PC12 cell plasma membranes were measured by atomic force microscopy in Kelvin probe mode (KPFM). The skewness values of the Φ_ss distribution histogram were found to be mostly negative, and the incorporation of negatively charged phosphatidylserine shifted the average skewness values to positive. After being treated with H₂O₂, dopamine, or Zn²⁺, phosphatidylserine was found to be translocated to the membrane outer leaflet and the averaged skewness values were changed to positive values. These results demonstrated that KPFM can be used to monitor cell physiology status in response to various stimulants with high spatial resolution.

Detection of uncharged or feebly charged small molecules by field-effect transistor biosensors

This paper describes a new technique for the detection of uncharged or feebly charged small molecules (<400Da) using Si field-effect transistor (FET) biosensors that are signal-enhanced by gold nanoparticle (NP) charges under dry measurement conditions. NP charges are quickly induced by a chemical deposition (that is, Au deposition) and the indirect competitive immunogold assay, and strongly enhance the electrical signals of the FET biosensors. For the validation of signal enhancement of FET biosensors based on NP charges and detection of uncharged or feebly charged small molecules, mycotoxins (MTXs) of aflatoxin-B1 (AFB1), zearalenone (ZEN), and ochratoxin-A (OTA) were used as target molecules. According to our experimental results, the signal is 100 times more enhanced than the use of the existing solution FET biosensing techniques. Furthermore, this method enables the FET biosensor to quantitatively detect target molecules, regardless of the ionic strengths, isoelectric points (pI), or pHs of the measured sample solutions.

Cathodoluminescence spectra of gallium nitride nanorods

Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.

Optimization of nanowire DNA sensor sensitivity using self-consistent simulation

In order to facilitate the rational design and the characterization of nanowire field-effect sensors, we have developed a model based on self-consistent charge-transport equations combined with interface conditions for the description of the biofunctionalized surface layer at the semiconductor/electrolyte interface. Crucial processes at the interface, such as the screening of the partial charges of the DNA strands and the influence of the angle of the DNA strands with respect to the nanowire, are computed by a Metropolis Monte Carlo algorithm for charged molecules at interfaces. In order to investigate the sensing mechanism of the device, we have computed the current–voltage characteristics, the electrostatic potential and the concentrations of electrons and holes. Very good agreement with measurements has been found and optimal device parameters have been identified. Our approach provides the capability to study the device sensitivity, which is of fundamental importance for reliable sensing.