Fabrication of a Horizontal and a Vertical Large Surface Area Nanogap Electrochemical Sensor

Nanogap sensors have a wide range of applications as they can provide accurate direct detection of biomolecules through impedimetric or amperometric signals. Signal response from nanogap sensors is dependent on both the electrode spacing and surface area. However, creating large surface area nanogap sensors presents several challenges during fabrication. We show two different approaches to achieve both horizontal and vertical coplanar nanogap geometries. In the first method we use electron-beam lithography (EBL) to pattern an 11 mm long serpentine nanogap (215 nm) between two electrodes. For the second method we use inductively-coupled plasma (ICP) reactive ion etching (RIE) to create a channel in a silicon substrate, optically pattern a buried 1.0 mm × 1.5 mm electrode before anodically bonding a second identical electrode, patterned on glass, directly above. The devices have a wide range of applicability in different sensing techniques with the large area nanogaps presenting advantages over other devices of the same family. As a case study we explore the detection of peptide nucleic acid (PNA)−DNA binding events using dielectric spectroscopy with the horizontal coplanar device

Nanogap capacitive biosensor for label-free aptamer-based protein detection

Recent advances in nanotechnology offer a new platform for the label free detection of biomolecules at ultra-low concentrations. Nano biosensors are emerging as a powerful method of improving device performance whilst minimizing device size, cost and fabrication times. Nanogap capacitive biosensors are an excellent approach for detecting biomolecular interactions due to the ease of measurement, low cost equipment needed and compatibility with multiplex formats.This thesis describes research into the fabrication of a nanogap capacitive biosensor and its detection results in label-free aptamer-based protein detection for proof of concept. Over the last four decades many research groups have worked on fabrication and applications of these type of biosensors, with different approaches, but there is much scope for the improvement of sensitivity and reliability. Additionally, the potential of these sensors for use in commercial markets and in everyday life has yet to be realized.Initial work in the field was limited to high frequency (>100 kHz) measurements only, since at low frequency there is significant electronic thermal noise (=4kBTR) from the electrical double layer (EDL). This was a significant drawback since this noise masked most of the important information from biomolecular interactions of interest. A novel approach to remove this parasitic noise is to minimize the EDL impedance by reducing the capacitor electrode separation to less than the EDL thickness. In the case of aptamer functionalized electrodes, this is particularly advantageous since device sensitivity is increased as the dielectric volume is better matched to the size of the biomolecules and their binding to the electrode surface. This work has demonstrated experimentally the concepts postulated theoretically.In this work we have fabricated a large area (100 x 5 μm x 5 μm) vertically oriented capacitive nanogap biosensor with a 40 nm electrode separation between two gold electrodes. A silicon dioxide support layer separates the two electrodes and this is partially etched (approximately 800 nm from both sides of each 5 μm x 5 μm capacitor), leaving an area of the gold electrodes available for thiol-aptamer functionalization.AC impedance spectroscopy measurements were performed with the biosensor in the presence of air, D.I. water, various ionic strength buffer solutions and aptamer/protein pairs inside the nanogap. Applied frequencies were from 1Hz to 500 kHz at 20 mV AC voltage with 0 DC. We obtained relative permittivity results as a function of frequency for air (ɛ=1) and DI water (ɛ~80) which compares very favorably with previous works done by different research groups.The sensitivity and response of the sensors to buffer solution (SSC buffer) with various ionic strengths (0.1x SSC, 0.2x SSC, 0.5x SSC and 1x SSC) was studied in detail. It was found that in the low frequency region (<1 kHz) the relative permittivity (capacitance) was broadly constant, that means it is independent from the applied frequency in this range. With increasing buffer concentration, the relative permittivity starts to increase (from ɛ=170 for 0.1x SSC to ɛ=260 for 1x SSC).The sensor performance was further investigated for aptamer-based protein detection, human alpha thrombin aptamers and human alpha thrombin protein pairs were selected for proof of concept. Aptamers were functionalized into the gold electrode surface with the Self-Assembly-Monolayer (SAM) method and measurements were performed in the presence of 0.5x SSC buffer solution (ɛ=180). Then the hybridization step was carried out with 1 μM of human alpha thrombin protein followed by measurements in the presence of the same buffer (ɛ=130). The response of the sensors with different solutions inside the nanogap was studied at room temperature (5 working devices were tested for each step). The replacement of the buffer solution (ɛ=250) with lower relative permittivity biomolecules (aptamer ɛ=180) and further binding proteins to immobilized aptamer (ɛ=130) was studied. To validate these results, a control experiment was carried out using different aptamers, in this case which are not able to bind to human alpha thrombin protein. It was found that the relative permittivity did not change after the hybridization step compared to the aptamer functionalization step, which indicates that the sensors performance is highly sensitive and reliable.This work serves as a proof of concept for a novel nanogap based biosensor with the potential to be used for many applications in environmental, food industry and medical industry. The fabrication method has been shown to be reliable and consistent with the possibility of being easily commercialized for mass production for use in laboratories for the analysis of a wide range of samples

Nanoscale Electrodes for Bionanosensing

Cancer is globally the second most common cause of death. Cancer burden rises to about 10 million deaths and more than 18 million new cases in 2018. Cancers are often diagnosed at a later stage preventing curative treatment. This underscores the need for an early stage diagnosis of cancer. Consequently, screening methods that can test patients’ samples taken by less invasive methods capable of early stage diagnosis are highly sought for. Based on this motivation, here we developed lab-on-a-chip diagnostic systems that can be used for early detection of cancer. Three different types of nanoscale electrodes were fabricated: (i) nanogap electrodes (ii) nano interdigitated electrodes and (iii) nanodisc electrodes and the possibility of using them for sensing and signal transduction were investigated. Chapter 2 describes the fabrication of nanogap device using conventional optical lithography and DNA detection across it using the electrical method. Chapter 3 details the fabrication of nano interdigitated electrodes (nIDEs) and their electrochemical validation. Chapter 4 describes the biosensing application of nIDEs using nanoparticle sandwich assay for the detection of DNA molecules. Chapter 5 describes the capturing of tdEVs on nIDEs, and its quantification using a sandwich immunosorbent assay on nIDEs. Chapter 6 proposes a new type of nanoscale electrodes which are termed as nanodisc electrodes. Chapter 7 explores the possibility of developing the nanodisc technology to a business idea. In short, the whole thesis tries to explore the different possibilities in developing a sensor that can be useful for cancer diagnosis

Integration of Biomolecular Recognition Elements with Solid-State Devices

Continued advances in stand-alone chemical sensors requires the introduction of new materials and transducers, and the seamless integration of the two. Electronic sensors represent one of the most efficient and versatile sensing transducers that offer advantages of high sensitivity, compatibility with multiple types of materials, network connectivity, and capability of miniaturization. With respect to materials to be used on this platform, many classes and subclasses of materials, including polymers, oxides, semiconductors, and composites have been investigated for various sensing environments. Despite numerous commercial products, major challenges remain. These include enhancing materials for selectivity/specificity, and low cost integration/ miniaturization of devices. Breakthroughs in either area would signify a transformative innovation. In this thesis, a combined materials and devices approach has been explored to address the above challenges. Biomolecular recognition elements, exemplified by aptamers, are the most recent addition to the library of tunable materials for specific detection of analytes. At the same time, nanoscale electrical devices based on tunnel junctions offer the potential for simple design, large scale integration, field deployment, network connectivity, and importantly, miniaturization to the molecular scale. To first establish a framework for studying sorption properties of solid oligonucleotides, custom designed aptamers sequences were studied to determine equilibrium partition coefficients. Linear-solvation-energy-relationship (LSER) analysis provides quantifications of non-covalent bonding properties and reveals the dominance of hydrogen bonding basicity in oligonucleotides. We find that DNA-analyte interactions have selective sorption properties similar to synthetic polymers. LSER analysis provides a chemical basis for material-analyte interactions. Oligonucleotide sequences were integrated with gold nanoparticle chemiresistors to transfer the selective sorption properties to microfabricated electrical devices. Responses generated by oligonucleotides under dry conditions were similar to standard organic mediums used as capping agents and suggests that DNA-based chemiresistor sensors operate with a similar mechanism based on sorption induced swelling. The equilibrium mass-sorption behavior of bulk DNA films could be translated to the chemiresistor sensitivity profiles. Our work establishes oligonucleotides, including aptamers, as a class of sorptive materials that can be systematically studied, engineered, and integrated with nanoscale electronic sensor devices. Experiments to investigate secondary structure effects were inconclusive and we conclude that further work should investigate DNA aptamers in buffered, aqueous environments to unequivocally establish the ability of chemiresitors to signal molecular recognition. Concurrent with the above studies, device integration and miniaturization was investigated to combine many sensing materials into a single, compact design. Arrays of nanoscale chemiresistors with critical features on the order of 10 – 100 nm were developed, using dielectrophoretic assembly of gold nanoparticles to control placement of the sensing material with nanometer accuracy. The nanoscale chemiresistors achieved the smallest known gold nanoparticle chemiresistors relying on just 2 – 3 layers of nanoparticles within 50 nm gaps, and were found to be more robust and less dependent on film thickness than previously published designs. Due to shorter diffusion paths, the sensors are also faster in response and recovery. A proof-of-concept, integrated single-chip sensor array was created and it showed similar response patterns as non-integrated sensor arrays. Dielectrophoresis is established as a key enabler for nanoscale, integrated devices. Based on the major findings of the thesis work, additional investigations were initiated to investigate the potential for nanoscale chemiresitor sensors to operate in buffered, aqueous (liquid) flow cells. Preliminary experiments show that chemiresistor sensing is transferable to liquid environments where analyte molecules are observed to partition from the bulk liquid to the sensing materials, leading to a detectable change of the device electrical properties. Comparing micron- and nano-scale devices fabricated using aqueous oligonucleotide-functionalized gold nanoparticles, it was found that nanoscale chemiresistors are more resistant to solvent damage than 5 µm chemiresistors. We conclude that future experiments to investigate aptamer sensing in aqueous solutions is a promising direction. Overall, this thesis is a significant contribution to materials development and device design to attain improved sensor selectivity and higher levels of device integration. First, it offers a scheme for design, selection, and validation of materials that confer analyte-specific interactions. Second, it paves the way for large scale sensor integration and parallel operation on a single chip. Lastly, it offers an approach to combine biomolecular recognition elements with electronic devices into robust, nanoscale detection systems. Based on the major findings of the thesis work, additional investigations were initiated to investigate the potential for nanoscale chemiresitor sensors to operate in buffered, aqueous (liquid) flow cells. Preliminary experiments show that chemiresistor sensing is transferable to liquid environments where analyte molecules are observed to partition from the bulk liquid to the sensing materials, leading to a detectable change of the device electrical properties. Comparing micron- and nano-scale devices fabricated using aqueous oligonucleotide-functionalized gold nanoparticles, it was found that nanoscale chemiresistors are more resistant to solvent damage than 5 µm chemiresistors. We conclude that future experiments to investigate aptamer sensing in aqueous solutions is a promising direction. Overall, this thesis is a significant contribution to materials development and device design to attain improved sensor selectivity and higher levels of device integration. First, it offers a scheme for design, selection, and validation of materials that confer analyte-specific interactions. Second, it paves the way for large scale sensor integration and parallel operation on a single chip. Lastly, it offers an approach to combine biomolecular recognition elements with electronic devices into robust, nanoscale detection systems

2d Suspended Fet Technology: Overcoming Scattering Effect For Ultrasensitive Reliable Biosensor

TMDs such as MoS2 is playing an important role in the field of FETs, photodetectors, thin film transistors and efficient biosensors because of their direct band-gap, high mobility, and biocompatibility. Despite these strengths, the performance and reliability of such atomic layer are easily influenced by supporting substrate. Interaction between the supporting substrate and MoS2 implies that interface control is vital for performance of devices consisting of monolayer MoS2. In particular, the Silicon dioxide (SiO2) supporting substrate has an uneven morphology and is chemically active because of trapped environmental gases, unknown functional groups, chemical adsorbates, and charges. Thus, adding another layer of MoS2 on the top of SiO2 cannot contribute charge transport clearly, which leads to the unreliable function of every single device. To solve the interface problem, suspended 2D layer devices have been reported by wet etching silicon di oxide underneath the monolayer. Freestanding MoS2 has shown 10 times greater back gate electronic mobility than the supporting on the SiO2 substrate. However, the existing SiO2 requires hazardous chemical etchants such as hydrofluoric acid (HF), which is difficult to handle and affects the 2D film structure and purity. Secondly, freestanding MoS2 sags between the two electrodes because of the high spacing (~ 2 µm), which makes it impossible to coat another layer such as hafnium oxide (HfO2) and antibodies on top of monolayer. Therefore, this structure impedes making top gate FET biosensors, which allows for only back gating. However, back gate mobility is far lesser than the top gate mobility which hinders making a highly sensitive FET-based biosensor because the sensitivity of a sensor depends on its mobility. In this work, CVD grown MoS2 channel material is transferred on self-assembled photolithographically patterned nano-gaps to achieve suspension and is covered with HfO2 to eliminate the direct functionalization of channel material. These nano-gap arrays provide mechanical strength to the monolayer and do not allow the supporting substrate to touch after coating another thin insulating layer as well as linkers/antibodies. HfO2 can be easily functionalized by silane-based linkers and antibodies (E-coli antibodies) to bring variation to the suspended 2D material by targeting a charged biomolecule (E-coli). In addition, termination of the supporting substrate leads to decrement of subthreshold swing which is inversly proportional to the sensitivity of the FET biosensor. The proposed FET biosensor has the capability to detect one molecule because of its single atomic layer as a channel material, its scalability due to the involvement of optical photolithography, and its fast response because of higher mobility

Nanocoax Arrays for Sensing Devices

Thesis advisor: Michael J. NaughtonWe have adapted a nanocoax array architecture for high sensitivity, all-electronic, chemical and biological sensing. Arrays of nanocoaxes with various dielectric annuli were developed using polymer replicas of Si nanopillars made via soft lithography. These arrays were implemented in the development of two different kinds of chemical detectors. First, arrays of nanocoaxes constructed with different porosity dielectric annuli were employed to make capacitive detectors for gaseous molecules and to investigate the role of dielectric porosity in the sensitivity of the device. Second, arrays of nanocoaxes with partially hollowed annuli were used to fabricate three-dimensional electrochemical biosensors within which we studied the role of nanoscale gap between electrodes on device sensitivity. In addition, we have employed a molecular imprint technique to develop a non-conducting molecularly imprinted polymer thin film of thickness comparable to size of biomolecules as an "artificial antibody" architecture for the detection of biomolecules.Thesis (PhD) — Boston College, 2014.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

DEVELOPING NANOPORE ELECTROMECHANICAL SENSORS WITH TRANSVERSE ELECTRODES FOR THE STUDY OF NANOPARTICLES/BIOMOLECULES

This study concerns development of a technology of utilizing metallic nanowires for a sensing element in nanofluidic single molecular (nanoparticle) sensors formed in plastic substrates to detect the translocation of single molecules through the nanochannel. We aimed to develop nanofluidic single molecular sensors in plastic substrates due to their scalability towards high through and low cost manufacturing for point-of-care applications. Despite significant research efforts recently on the technologies and applications of nanowires, using individual nanowires as electric sensing element in nanofluidic bioanalytic devices has not been realized yet. This dissertation work tackles several technical challenges involved in this development, which include reduction of nanowire agglomerates in the deposition of individual nanowires on a substrate, large scale alignment/assembly of metallic nanowires, placement of single nanowires on microelectrodes, characterization of electrical conductance of single nanowire, bonding of a cover plate to a substrate with patterned microelectrodes and nanowire electrodes. Overcoming the abovementioned challenges, we finally demonstrated a nanofluidic sensor with an in-plane nanowire electrode in poly(methyl methacrylate) substrates for sensing single biomolecules. In the first part of this study, we developed the processes for separation and large-scale assembly of individual NiFeCo nanowires grown using an electrodeposition process inside a porous alumina template. A method to fabricate microelectrode patterns on plastic substrates using flexible stencil masks was developed. We studied electrical and magnetic properties of new composite core-shell nanowires by measuring the electrical transport through individual nanowires. The core-shell nanowires were composed of a mechanically stable FeNiCo core and an ultrathin shell of a highly conductive Au gold (FeNiCo-Au nanowires). In the second part of this study, we simulated the effects of the nanopore geometry on the current drop signal of the translocation through a nanopore via finite element method using COMSOL. Using the above techniques, we developed for the fabrication and alignment of the microelectrodes and nanowires, we studied the optimum conditions to integrate the transverse nanoelectrode with the nanochannel on plastic substrates. The main challenge was to find the conditions to embed the micro-/nanoelectrodes into the nanochannel substrate as well as the nanochannel cover sheet

Fabrication and characterization of nanostructured porous silicon-silver composite layers by cyclic deposition: Dip-coating vs spin-coating

“This is the Accepted Manuscript version of an article accepted for publication in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6528/ab96e5”Composites of nanostructured porous silicon and silver (nPSi-Ag) have attracted great attention due to the wide spectrum of applications in fields such as microelectronics, photonics, photocatalysis and bioengineering, Among the different methods for the fabrication of nanostructured composite materials, dip and spin-coating are simple, versatile, and cost-effective bottom-up technologies to provide functional coatings. In that sense, we aimed at fabricating nPSi-Ag composite layers. Using nPSi layers with pore diameter of 30 nm, two types of thin-film techniques were systematically compared: cyclic dip-coating (CDC) and cyclic spin-coating (CSC). CDC technique formed a mix of granular and flake-like structures of metallic Ag, and CSC method favored the synthesis of flake-like structures with Ag and Ag2O phases. Flakes obtained by CDC and CSC presented a width of 110 nm and 70 nm, respectively. Particles also showed a nanostructure surface with features around 25 nm. According to the results of EDX and RBS, integration of Ag into nPSi was better achieved using the CDC technique. SERS peaks related to chitosan adsorbed on Ag nanostructures were enhanced, especially in the nPSi-Ag composite layers fabricated by CSC compared to CDC, which was confirmed by FTDT simulations. These results show that CDC and CSC produce different nPSi-Ag composite layers for potential applications in bioengineering and photonicsThis work was financially supported by Fondo Nacional de Desarrollo Científico y Tecnológico FONDECYT–Chile (grant number 11180395), FONDEQUIP–Chile (project 160152