Direct electrical detection of antigen–antibody binding on diamond and silicon substrates using electrical impedance spectroscopy

Diamonds for Life: Developments in Sensors for Biomolecules

Diamond-based electrodes and biosensors are interesting in analytics because of their particular set of properties, namely: large potential window, chemical inertness, low baseline current, stability, and transparency. Diamond-based electrodes and biosensors were shown to detect biological molecules such as neurotransmitters and proteins, respectively. In this review, we summarise the different types of diamond electrodes and biosensors based on their type of detection (electrochemical or optical), functionalisation, and target analyte. The last section presents a discussion on the different analytical responses obtained with electrodes or biosensors, according to the type of analyte. Electrodes work quite well for detecting small molecules with redox properties, whereas biosensors are more suited for detecting molecules with a high molecular weight, such as DNA and proteins.

[Methods for assessing the electrophysical properties of body tissues and the possibility of their application in forensic medical practice]

This article provides an overview of some diagnostic methods for indicators and changes in the electrophysical properties of biological tissues. The key principles of assessing the electrical conductivity and resistance of tissues under the influence of alternating and direct current electric charge using standard and modified meters and signal generators, as well as the possibilities of digital improvement of computational models, are considered. Some existing and promising methods are presented that enable to solve important problems of forensic medical practice by registering changes in electrophysical parameters.

Photoluminescence label-free immunosensor for the detection of Aflatoxin B1 using polyacrylonitrile/zinc oxide nanofibers

The precise and rapid detection of hazardous molecules, microorganisms, pollutants, and toxins currently remains a global challenge. Aflatoxin B1 (AFB1) is a toxic and dangerous product of fungi that considered as cancerogenic, mutagenic, and immunosuppressive for humans and animals. Therefore, the screening of AFB1 in food and beverages plays an important role in preventing foodborne illnesses. In this study, AFB1 molecules were detected in a microfluidic device with integrated polyacrylonitrile/zinc oxide (PAN/ZnO) nanofibers fabricated via a combination of the electrospinning, and atomic layer deposition (ALD) techniques. The structural and optical analyses of PAN/ZnO nanofibers were performed and samples with the most suitable properties were utilized for AFB1 detection. In order to obtain the biorecognition layer towards AFB1, PAN/ZnO samples were modified by (3-Aminopropyl) triethoxysilane (APTES), and glutaraldehyde (GA), bovine serum albumin (BSA) and monoclonal antibodies (Anti-AFB1). Subsequently, photoluminescence (PL)-based immunosensor was integrated into a microfluidic cell and tested for AFB1 detection. The mechanism of PL changes caused by AFB1 & Anti-AFB1 complex formation was analyzed and developed. The proposed approach enables the detection of AFB1 with the lowest concentration (LOD) of about 39 pg/ml, while the sensitivity range was evaluated as 0.1-20 ng/ml. The obtained values of LOD and sensitivity, as well as the simplicity of the detection method, make this approach a prospect for further application.

Deoxyribonucleic-acid-sensitive Polycrystalline Diamond Solution-gate Field-effect Transistor with a Carboxyl-terminated Boron-doped Channel

This paper describes a deoxyribonucleic-acid-sensitive electrolyte solution-gate field-effect transistor (SGFET) sensor utilizing a partial carboxyl-terminated boron-doped polycrystalline diamond surface as a linker to connect a deoxyribonucleic acid (DNA) probe. A high density of carboxyl termination on the polycrystalline diamond surface that was employed as a FET channel was achieved using a vacuum ultraviolet system with oxygen gas. A single-stranded DNA probe was immobilized on the polycrystalline diamond channel via amino coupling. The current-voltage characteristics of the polycrystalline diamond SGFET sensor was examined with bias voltages within its potential voltage window. The characteristics of the drain-source current verses the drain-source voltage showed a pinch-off, a shift voltage of up to 40 mV with a coefficient of variation of 4 - 11% was obtained between hybridization and denaturation. In addition, a single nucleotide mutation of DNA sequence was selectively recognized by the shift voltage up to ca. 10 mV.

EIS-Based Biosensors in Foodborne Pathogen Detection with a Special Focus on Listeria monocytogenes

In this chapter methods and protocols for surfaces adapted to electrochemical impedance detection, antibody binding, electrolyte couples used, and instrumentation for EIS Biosensing are presented. Various technical bottlenecks have been overcome in recent years. Other limitations still present in this technique are discussed. We present the most recent applications in food pathogen detection based on EIS methods, as well as using other antibody-based platforms.

Fluorescence-Free Biosensor Methods in Detection of Food Pathogens with a Special Focus on Listeria monocytogenes

Food pathogens contaminate food products that allow their growth on the shelf and also under refrigerated conditions. Therefore, it is of utmost importance to lower the limit of detection (LOD) of the method used and to obtain the results within hours to few days. Biosensor methods exploit the available technologies to individuate and provide an approximate quantification of the bacteria present in a sample. The main bottleneck of these methods depends on the aspecific binding to the surfaces and on a change in sensitivity when bacteria are in a complex food matrix with respect to bacteria in a liquid food sample. In this review, we introduce surface plasmon resonance (SPR), new advancements in SPR techniques, and electrochemical impedance spectroscopy (EIS), as fluorescence-free biosensing technologies for detection of L. monocytogenes in foods. The application of the two methods has facilitated L. monocytogenes detection with LOD of 1 log CFU/mL. Further advancements are envisaged through the combination of biosensor methods with immunoseparation of bacteria from larger volumes, application of lab-on-chip technologies, and EIS sensing methods for multiplex pathogen detection. Validation efforts are being conducted to demonstrate the robustness of detection, reproducibility and variability in multi-site installations.

Impedance Biosensors: Applications to Sustainability and Remaining Technical Challenges

Due to their all-electrical nature, impedance biosensors have significant potential for use as simple and portable sensors for environmental studies and environmental monitoring. Detection of two endocrine-disrupting chemicals (EDC), norfluoxetine and BDE-47, is reported here by impedance biosensing, with a detection limit of 8.5 and 1.3 ng/mL for norfluoxetine and BDE-47, respectively. Although impedance biosensors have been widely studied in the academic literature, commercial applications have been hindered by several technical limitations, including possible limitations to small analytes, the complexity of impedance detection, susceptibility to nonspecific adsorption, and stability of biomolecule immobilization. Recent research into methods to overcome these obstacles is briefly reviewed. New results demonstrating antibody regeneration atop degenerate (highly doped) Si are also reported. Using 0.2 M KSCN and 10 mM HF for antibody regeneration, peanut protein Ara h 1 is detected daily during a 30 day trial.

Bioelectrical Impedance Methods for Noninvasive Health Monitoring: A Review

Under the alternating electrical excitation, biological tissues produce a complex electrical impedance which depends on tissue composition, structures, health status, and applied signal frequency, and hence the bioelectrical impedance methods can be utilized for noninvasive tissue characterization. As the impedance responses of these tissue parameters vary with frequencies of the applied signal, the impedance analysis conducted over a wide frequency band provides more information about the tissue interiors which help us to better understand the biological tissues anatomy, physiology, and pathology. Over past few decades, a number of impedance based noninvasive tissue characterization techniques such as bioelectrical impedance analysis (BIA), electrical impedance spectroscopy (EIS), electrical impedance plethysmography (IPG), impedance cardiography (ICG), and electrical impedance tomography (EIT) have been proposed and a lot of research works have been conducted on these methods for noninvasive tissue characterization and disease diagnosis. In this paper BIA, EIS, IPG, ICG, and EIT techniques and their applications in different fields have been reviewed and technical perspective of these impedance methods has been presented. The working principles, applications, merits, and demerits of these methods has been discussed in detail along with their other technical issues followed by present status and future trends.

Nanostructuring of biosensing electrodes with nanodiamonds for antibody immobilization

While chemical vapor deposition of diamond films is currently cost prohibitive for biosensor construction, in this paper, we show that sonication-assisted nanostructuring of biosensing electrodes with nanodiamonds (NDs) allows harnessing the hydrolytic stability of the diamond biofunctionalization chemistry for real-time continuous sensing, while improving the detector sensitivity and stability. We find that the higher surface coverages were important for improved bacterial capture and can be achieved through proper choice of solvent, ND concentration, and seeding time. A mixture of methanol and dimethyl sulfoxide provides the highest surface coverage (33.6 ± 3.4%) for the NDs with positive zeta-potential, compared to dilutions of dimethyl sulfoxide with acetone, ethanol, isopropyl alcohol, or water. Through impedance spectroscopy of ND-seeded interdigitated electrodes (IDEs), we found that the ND seeds serve as electrically conductive islands only a few nanometers apart. Also we show that the seeded NDs are amply hydrogenated to be decorated with antibodies using the UV-alkene chemistry, and higher bacterial captures can be obtained compared to our previously reported work with diamond films. When sensing bacteria from 10(6) cfu/mL E. coli O157:H7, the resistance to charge transfer at the IDEs decreased by ∼ 38.8%, which is nearly 1.5 times better than that reported previously using redox probes. Further in the case of 10(8) cfu/mL E. coli O157:H7, the charge transfer resistance changed by ∼ 46%, which is similar to the magnitude of improvement reported using magnetic nanoparticle-based sample enrichment prior to impedance detection. Thus ND seeding allows impedance biosensing in low conductivity solutions with competitive sensitivity.

Nanosensor electrical immunoassay for quantitative detection of NT-pro brain natriuretic peptide

AIM

To demonstrate a label-free electrical immunoassay for profiling vascular biomarker N-terminal pro-brain natriuretic peptide (NT-proBNP) associated with improved cardiac risk prediction.

MATERIALS & METHODS

A high-density nanowell-based electrical immunoassay has been designed by integrating nanoporous aluminum oxide onto printed circuit board chips for the detection of NT-proBNP. The concentration of the biomarker is quantitatively determined by measuring impedance changes to the electrical double layer within the nanowells using electrochemical impedance spectroscopy. Detection sensitivity in the fg/ml range was obtained due to spatial confinement of the target biomarkers in size-matched nanowells.

RESULTS & DISCUSSION

Electrical immunoassay performance was determined for the detection of NT-proBNP in phosphate-buffered saline (PBS) and human serum (HS). The lower limit of detection for the sensor was observed to be 10 fg/ml in PBS and 500 fg/ml in HS. The upper limit of detection was observed to be 500 fg/ml in PBS and 500 ng/ml in HS.

CONCLUSION

A label-free technique for detection of NT-proBNP at clinically relevant concentrations for evaluating cardiac risk is demonstrated. High sensitivity and specificity, robust detection and low volume (100 µl) per assay project the technology to be a successful competitor to traditional ELISA-based techniques.

Chemical passivation processes for biofunctionalization schemes on semiconductor surfaces

In developing novel designs for semiconductor-based biosensors and for biofunctionalization of semiconductors in general, it is extremely important to be able to block the reaction sites present on a surface following the biomodification from further chemical transformations. This procedure is required both to protect the surface from oxidation and to allow for molecular-level control of the biomolecular interactions at the topmost layer. In this work, the biosensor model system is designed based on a single-strand biotin-modified thiol-DNA attached to the silicon substrate. The binding of this thiol-DNA to the surface is performed through the cross-linker sulfosuccinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SSMCC) attached to the 11-amino-1-undecene monolayer on Si(111) surface. Streptavidin-coated gold nanoparticles are used to test the reactivity of the surface and to examine the role of passivation in the entire scheme. The passivation of the remaining surface reactive sites is achieved via a reaction with 1-octadecanethiol (ODT). This approach tests both the stability of the silicon/organic layer interface and the passivation of the biofunctionalized surface on top of the organic layer. Microscopy and spectroscopy studies are combined to interrogate this model system before and after surface passivation.

Polycrystalline-Diamond MEMS Biosensors Including Neural Microelectrode-Arrays

Diamond is a material of interest due to its unique combination of properties, including its chemical inertness and biocompatibility. Polycrystalline diamond (poly-C) has been used in experimental biosensors that utilize electrochemical methods and antigen-antibody binding for the detection of biological molecules. Boron-doped poly-C electrodes have been found to be very advantageous for electrochemical applications due to their large potential window, low background current and noise, and low detection limits (as low as 500 fM). The biocompatibility of poly-C is found to be comparable, or superior to, other materials commonly used for implants, such as titanium and 316 stainless steel. We have developed a diamond-based, neural microelectrode-array (MEA), due to the desirability of poly-C as a biosensor. These diamond probes have been used for in vivo electrical recording and in vitro electrochemical detection. Poly-C electrodes have been used for electrical recording of neural activity. In vitro studies indicate that the diamond probe can detect norepinephrine at a 5 nM level. We propose a combination of diamond micro-machining and surface functionalization for manufacturing diamond pathogen-microsensors.

Tandem "click" reactions at acetylene-terminated Si(100) monolayers

We demonstrate a simple method for coupling alkynes to alkynes. The method involves tandem azide-alkyne cycloaddition reactions ("click" chemistry) for the immobilization of 1-alkyne species onto an alkyne modified surface in a one-pot procedure. In the case presented, these reactions take place on a nonoxidized Si(100) surface although the approach is general for linking alkynes to alkynes. The applicability of the method in the preparation of electrically well-behaved functionalized surfaces is demonstrated by coupling an alkyne-tagged ferrocene species onto alkyne-terminated Si(100) surfaces. The utility of the approach in biotechnology is shown by constructing a DNA sensing interface by derivatization of the acetylenyl surface with commercially available alkyne-tagged oligonucleotides. Cyclic voltametry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and X-ray reflectometry are used to characterize the coupling reactions and performance of the final modified surfaces. These data show that this synthetic protocol gives chemically well-defined, electronically well-behaved, and robust (bio)functionalized monolayers on silicon semiconducting surfaces.

Control of Nanoscale Environment to Improve Stability of Immobilized Proteins on Diamond Surfaces

Immunoassays for detection of bacterial pathogens rely on the selectivity and stability of bio-recognition elements such as antibodies tethered to sensor surfaces. The search for novel surfaces that improve the stability of biomolecules and assay performance has been pursued for a long time. However, the anticipated improvements in stability have not been realized in practice under physiological conditions because the surface functionalization layers on commonly used substrates, silica and gold, are themselves unstable on time scales of days. In this paper, we show that covalent linking of antibodies to diamond surfaces leads to substantial improvements in biological activity of proteins as measured by the ability to selectively capture cells of the pathogenic bacterium Escherichia coli O157:H7 even after exposure to buffer solutions at 37 °C for extended periods of time, approaching 2 weeks. Our results from ELISA, XPS, fluorescence microscopy, and MD simulations suggest that by using highly stable surface chemistry and controlling the nanoscale organization of the antibodies on the surface, it is possible to achieve significant improvements in biological activity and stability. Our findings can be easily extended to functionalization of micro and nanodimensional sensors and structures of biomedical diagnostic and therapeutic interest.

Surface functionalization of thin-film diamond for highly stable and selective biological interfaces

Carbon is an extremely versatile family of materials with a wide range of mechanical, optical, and mechanical properties, but many similarities in surface chemistry. As one of the most chemically stable materials known, carbon provides an outstanding platform for the development of highly tunable molecular and biomolecular interfaces. Photochemical grafting of alkenes has emerged as an attractive method for functionalizing surfaces of diamond, but many aspects of the surface chemistry and impact on biological recognition processes remain unexplored. Here we report investigations of the interaction of functionalized diamond surfaces with proteins and biological cells using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and fluorescence methods. XPS data show that functionalization of diamond with short ethylene glycol oligomers reduces the nonspecific binding of fibrinogen below the detection limit of XPS, estimated as > 97% reduction over H-terminated diamond. Measurements of different forms of diamond with different roughness are used to explore the influence of roughness on nonspecific binding onto H-terminated and ethylene glycol (EG)-terminated surfaces. Finally, we use XPS to characterize the chemical stability of Escherichia coli K12 antibodies on the surfaces of diamond and amine-functionalized glass. Our results show that antibody-modified diamond surfaces exhibit increased stability in XPS and that this is accompanied by retention of biological activity in cell-capture measurements. Our results demonstrate that surface chemistry on diamond and other carbon-based materials provides an excellent platform for biomolecular interfaces with high stability and high selectivity.

Biofunctionalization on alkylated silicon substrate surfaces via "click" chemistry

Biofunctionalization of silicon substrates is important to the development of silicon-based biosensors and devices. Compared to conventional organosiloxane films on silicon oxide intermediate layers, organic monolayers directly bound to the nonoxidized silicon substrates via Si-C bonds enhance the sensitivity of detection and the stability against hydrolytic cleavage. Such monolayers presenting a high density of terminal alkynyl groups for bioconjugation via copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC, a "click" reaction) were reported. However, yields of the CuAAC reactions on these monolayer platforms were low. Also, the nonspecific adsorption of proteins on the resultant surfaces remained a major obstacle for many potential biological applications. Herein, we report a new type of "clickable" monolayers grown by selective, photoactivated surface hydrosilylation of α,ω-alkenynes, where the alkynyl terminal is protected with a trimethylgermanyl (TMG) group, on hydrogen-terminated silicon substrates. The TMG groups on the film are readily removed in aqueous solutions in the presence of Cu(I). Significantly, the degermanylation and the subsequent CuAAC reaction with various azides could be combined into a single step in good yields. Thus, oligo(ethylene glycol) (OEG) with an azido tag was attached to the TMG-alkyne surfaces, leading to OEG-terminated surfaces that reduced the nonspecific adsorption of protein (fibrinogen) by >98%. The CuAAC reaction could be performed in microarray format to generate arrays of mannose and biotin with varied densities on the protein-resistant OEG background. We also demonstrated that the monolayer platform could be functionalized with mannose for highly specific capturing of living targets (Escherichia coli expressing fimbriae) onto the silicon substrates.

Surface chemistry for stable and smart molecular and biomolecular interfaces via photochemical grafting of alkenes

Many emerging fields such as biotechnology and renewable energy require functionalized surfaces that are "smart" and highly stable. Surface modification schemes developed previously have often been limited to simple molecules or have been based on weakly bound layers that have limited stability. In this Account, we report on recent developments enabling the preparation of molecular and biomolecular interfaces that exhibit high selectivity and unprecedented stability on a range of covalent materials including diamond, vertically aligned carbon nanofibers, silicon, and metal oxides. One particularly successful pathway to ultrastable interfaces involves the photochemical grafting of organic alkenes to the surfaces. Bifunctional alkenes with a suitable functional group at the distal end can directly impart functionality and can serve as attachment points for linking complex structures such as DNA and proteins. The successful application of photochemical grafting to a surprisingly wide range of materials has motivated researchers to better understand the underlying photochemical reaction mechanisms. The resulting studies using experimental and computational methods have provided fundamental insights into the electronic structure of the molecules and the surface control photochemical reactivity. Such investigations have revealed the important role of a previously unrecognized process, photoelectron emission, in initiating photochemical grafting of alkenes to surfaces. Molecular and biomolecular interfaces formed on diamond and other covalent materials are leading to novel types of molecular electronic interfaces. For example, electrical, optical, or electromechanical structures that convert biological information directly into analytical signals allow for direct label-free detection of DNA and proteins. Because of the preferential adherence of molecules to graphitic edge-plane sites, the grafting of redox-active species to vertically aligned carbon nanofibers leads to good electrochemical activity. Therefore researchers could graft electrocatalytic materials to carbon nanofibers to develop new types of selective electrocatalytic interfaces. Extending this chemistry to include metal oxides such as TiO(2) may lead to highly specific and efficient chemical reactions and new materials with useful applications in photovoltaic and photocatalytic energy conversion.

Biogenic nanoporous silica-based sensor for enhanced electrochemical detection of cardiovascular biomarkers proteins

The goal of our research is to demonstrate the feasibility of employing biogenic nanoporous silica as a key component in developing a biosensor platform for rapid label-free electrochemical detection of cardiovascular biomarkers from pure and commercial human serum samples with high sensitivity and selectivity. The biosensor platform consists of a silicon chip with an array of gold electrodes forming multiple sensor sites and works on the principle of electrochemical impedance spectroscopy. Each sensor site is overlaid with a biogenic nanoporous silica membrane that forms a high density of nanowells on top of each electrode. When specific protein biomarkers: C-reactive protein (CRP) and myeloperoxidase (MPO) from a test sample bind to antibodies conjugated to the surface of the gold surface at the base of each nanowell, a perturbation of electrical double layer occurs resulting in a change in the impedance. The performance of the biogenic silica membrane biosensor was tested in comparison with nanoporous alumina membrane-based biosensor and plain metallic thin film biosensor. Significant enhancement in the sensitivity and selectivity was achieved with the biogenic silica biosensor, in comparison to the other two, for detecting the two protein biomarkers from both pure and commercial human serum samples. The sensitivity of the biogenic silica biosensor is approximately 1 pg/ml and the linear dose response is observed over a large dynamic range from 1 pg/ml to 1 microg/ml. Based on its performance metrics, the biogenic silica biosensor has excellent potential for development as a point of care handheld electronic biosensor device for detection of protein biomarkers from clinical samples.

Structural and optical properties of DNA layers covalently attached to diamond surfaces

Label-free detection of DNA molecules on chemically vapor-deposited diamond surfaces is achieved with spectroscopic ellipsometry in the infrared and vacuum ultraviolet range. This nondestructive method has the potential to yield information on the average orientation of single as well as double-stranded DNA molecules, without restricting the strand length to the persistence length. The orientational analysis based on electronic excitations in combination with information from layer thicknesses provides a deeper understanding of biological layers on diamond. The pi-pi* transition dipole moments, corresponding to a transition at 4.74 eV, originate from the individual bases. They are in a plane perpendicular to the DNA backbone with an associated n-pi* transition at 4.47 eV. For 8-36 bases of single- and double-stranded DNA covalently attached to ultra-nanocrystalline diamond, the ratio between in- and out-of-plane components in the best fit simulations to the ellipsometric spectra yields an average tilt angle of the DNA backbone with respect to the surface plane ranging from 45 degrees to 52 degrees . We comment on the physical meaning of the calculated tilt angles. Additional information is gathered from atomic force microscopy, fluorescence imaging, and wetting experiments. The results reported here are of value in understanding and optimizing the performance of the electronic readout of a diamond-based label-free DNA hybridization sensor.

Direct electrical transduction of antibody binding to a covalent virus layer using electrochemical impedance

Electrochemical impedance spectroscopy is used to detect the binding of a 148.2 kDa antibody to a "covalent virus layer" (CVL) immobilized on a gold electrode. The CVL consisted of M13 phage particles covalently anchored to a 3 mm diameter gold disk electrode. The ability of the CVL to distinguish this antibody ("p-Ab") from a second, nonbinding antibody ("n-Ab") was evaluated as a function of the frequency and phase of the measured current relative to the applied voltage. The binding of p-Ab to the CVL was correlated with a change in the resistance, reducing it at low frequency (1-40 Hz) while increasing it at high frequency (2-140 kHz). The capacitance of the CVL was virtually uncorrelated with p-Ab binding. At both low and high frequency, the electrode resistance was linearly dependent on the p-Ab concentration from 20 to 266 nM but noise compromised the reproducibility of the p-Ab measurement at frequencies below 40 Hz. A "signal-to-noise" ratio for antibody detection was computed based upon the ratio between the measured resistance change upon p-Ab binding and the standard deviation of this change obtained from multiple measurements. In spite of the fact that the impedance change upon p-Ab binding in the low frequency domain was more than 100 times larger than that measured at high frequency, the S/N ratio at high frequency was higher and virtually independent of frequency from 4 to 140 kHz. Attempts to release p-Ab from the CVL using 0.05 M HCl, as previously described for mass-based detection, caused a loss of sensitivity that may be associated with a transition of these phage particles within the CVL from a linear to a coiled conformation at low pH.

Formation and characterization of organic monolayers on semiconductor surfaces

Organic-semiconductor interfaces are playing increasingly important roles in fields ranging from electronics to nanotechnology to biosensing. The continuing decrease in microelectronic device feature sizes is raising an especially great interest in understanding how to integrate molecular systems with conventional, inorganic microelectronic materials, particularly silicon. The explosion of interest in the biological sciences has provided further impetus for learning how to integrate biological molecules and systems with microelectronics to form true bioelectronic systems. Organic monolayers present an excellent opportunity for surmounting many of the practical barriers that have hindered the full integration of microelectronics technology with organic and biological systems. Of all the semiconductor materials, silicon and diamond stand out as unique. This review focuses upon the preparation and characterization of organic and biomolecular layers on semiconductor surfaces, with special emphasis on monolayers formed on silicon and diamond.