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The 2018 Nobel Prize in Chemistry recognized in vitro evolution, including the development by George Smith and Gregory Winter of phage display, a technology for engineering the functional capabilities of antibodies into viruses. Such bacteriophages solve inherent problems with antibodies, including their high cost, thermal lability, and propensity to aggregate. While phage display accelerated the discovery of peptide and protein motifs for recognition and binding to proteins in a variety of applications, the development of biosensors using intact phage particles was largely unexplored in the early 2000s. Virus particles, 16.5 MDa in size and assembled from thousands of proteins, could not simply be substituted for antibodies in any existing biosensor architectures.Incorporating viruses into biosensors required us to answer several questions: What process will allow the incorporation of viruses into a functional bioaffinity layer? How can the binding of a protein disease marker to a virus particle be electrically transduced to produce a signal? Will the variable salt concentration of a bodily fluid interfere with electrical transduction? A completely new biosensor architecture and a new scheme for electrical transduction of the binding of molecules to viruses were required.This Account describes the highlights of a research program launched in 2006 that answered these questions. These efforts culminated in 2018 in the invention of a biosensor specifically designed to interface with virus particles: the Virus BioResistor (VBR). The VBR is a resistor consisting of a conductive polymer matrix in which M13 virus particles are entrained. The electrical impedance of this resistor, measured across 4 orders of magnitude in frequency, simultaneously measures the concentration of a target protein and the ionic conductivity of the medium in which the resistor is immersed. Large signal amplitudes coupled with the inherent simplicity of the VBR sensor design result in high signal-to-noise ratio (S/N > 100) and excellent sensor-to-sensor reproducibility. Using this new device, we have measured the urinary bladder cancer biomarker nucleic acid deglycase (DJ-1) in urine samples. This optimized VBR is characterized by extremely low sensor-to-sensor coefficients of variation in the range of 3-7% across the DJ-1 binding curve down to a limit of quantitation of 30 pM, encompassing 4 orders of magnitude in concentration.

Conspectus Hydrogen gas (H2) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to "sniff" for leaked H2 wherever it is used. Electrical H2 sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H2 causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdHx). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H2 safety sensing. In this Account, we describe the development of H2 sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H2 sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10-15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H2 exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H2 chemisorption and desorption of H2 were rate-limiting steps in sensor response to and recovery from H2 exposure. Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H2 exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H2 exposure. H2 dissociatively chemisorbs at a Pt surface to form Pt-H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H2 exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H2 concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H2 while retaining the same high sensitivity to H2 concentration seen for sensors based upon pure Pd nanowires. A new chapter in H2 sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H2. [ABSTRACT FROM AUTHOR]

Hydrogen gas (H 2 ) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to "sniff" for leaked H 2 wherever it is used. Electrical H 2 sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H 2 causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdH x ). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H 2 safety sensing. In this Account, we describe the development of H 2 sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H 2 sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10-15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H 2 exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H 2 chemisorption and desorption of H 2 were rate-limiting steps in sensor response to and recovery from H 2 exposure. Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H 2 exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H 2 exposure. H 2 dissociatively chemisorbs at a Pt surface to form Pt-H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H 2 exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H 2 concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H 2 while retaining the same high sensitivity to H 2 concentration seen for sensors based upon pure Pd nanowires. A new chapter in H 2 sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H 2 .