Deep Learning Image Analysis of Nanoplasmonic Sensors: Toward Medical Breath Monitoring.

Sensing biomarkers in exhaled breath offers a potentially portable, cost-effective, and noninvasive strategy for disease diagnosis screening and monitoring, while high sensitivity, wide sensing range, and target specificity are critical challenges. We demonstrate a deep learning-assisted plasmonic sensing platform that can detect and quantify gas-phase biomarkers in breath-related backgrounds of varying complexity. The sensing interface consisted of Au/SiO2 nanopillars covered with a 15 nm metal-organic framework. A small camera was utilized to capture the plasmonic sensing responses as images, which were subjected to deep learning signal processing. The approach has been demonstrated at a classification accuracy of 95 to 98% for the diabetic ketosis marker acetone within a concentration range of 0.5-80 µmol/mol. The reported work provides a thorough exploration of single-sensor capabilities and sets the basis for more advanced utilization of artificial intelligence in sensing applications.

Conformational Changes of Immobilized Polythymine due to External Stressors Studied with Temperature-Controlled Electrochemical Microdevices.

Conformational changes of single-stranded DNA (ssDNA) play an important role in a DNA strand's ability to bind to target ligands. A variety of factors can influence conformation, including temperature, ionic strength, pH, buffer cation valency, strand length, and sequence. To better understand the effects of these factors on immobilized DNA structures, we employ temperature-controlled electrochemical microsensors to study the effects of salt concentration and temperature variation on the conformation and motion of polythymine (polyT) strands of varying lengths (10, 20, 50 nucleotides). PolyT strands were tethered to a gold working electrode at the proximal end through a thiol linker via covalent bonding between the Au electrode and sulfur link, which can tend to decompose between a temperature range of 60 and 90 °C. The strands were also modified with an electrochemically active methylene blue (MB) moiety at the distal end. Electron transfer (eT) was measured by square wave voltammetry (SWV) and used to infer information pertaining to the average distance between the MB and the working electrode. We observe changes in DNA flexibility due to varying ionic strength, while the effects of increased DNA thermal motion are tracked for elevated temperatures. This work elucidates the behavior of ssDNA in the presence of a phosphate-buffered saline at NaCl concentrations ranging from 20 to 1000 mmol/L through a temperature range of 10-50 °C in 1° increments, well below the decomposition temperature range. The results lay the groundwork for studies on more complex DNA strands in conjunction with different chemical and physical conditions.

Miniaturized nanohole array based plasmonic sensor for the detection of acetone and ethanol with insights into the kinetics of adsorptive plasmonic sensing.

The present work demonstrates development of a miniaturized plasmonic platform comprised of a Au nanohole array (NHA) on a Si/Si3N4 substrate. Plasmonic responses of the NHA platform, which is coated with Cu-benzenetricarboxylate metal organic framework (MOF), are found to be promising even towards 500 nmol mol-1 (ppb) of acetone or ethanol vapors at room temperature. The sensing characteristics are further investigated by varying the operating temperature (296 K to 318 K) of the sensor and the concentrations of vapors (500 nmol mol-1 to 320 µmol mol-1). The plasmonic responses for the sensors are correlated with the adsorption of vapors on the MOF surface and modeled in accordance to Langmuir-type adsorption. Kinetic parameters are estimated for the adsorption of fixed concentrations of acetone and ethanol vapors within the studied operating temperature range. The linear variation of characteristic response time constants with the operating temperature provides Arrhenius activation energies for the adsorption of acetone and ethanol vapors. The comparatively lower activation energy estimated for the adsorption of ethanol results in faster and more sensitive response of the sensor towards that analyte. The plasmonic sensor for the detection of nmol mol-1 level acetone and ethanol vapors at room temperature along with the kinetic correlation on plasmonic response with the adsorption of the analytes described herein offer new insights to existing reports on surface modification and plasmonic detection.

Dithiol-based modification of poly(dopamine): enabling protein resistance via short-chain ethylene oxide oligomers.

We present a facile strategy to modify poly(dopamine) (PDA)-coated substrates. Using thiol-terminated short chain ethylene oxide oligomers (OEG) under aqueous conditions, we explore the creation of a model surface exhibiting resistance to nonspecific protein absorption (RPA) by engineering the surface properties of a PDA adlayer. Surprisingly, dithiol-terminated OEG molecules demonstrated significantly greater coverage on PDA surfaces than analogous monothiol molecules. Successful RPA is only achieved with dithiol-terminated OEGs.

A generalized strategy for immobilizing uniformly oriented membrane proteins at solid interfaces.

We have developed a method based on self-assembly of thiols on Au substrates to immobilize membrane proteins at interfaces. Using water soluble nitrilotriacetic acid (NTA)-terminated oligo(ethylene glycol) thiols, a histidine-tagged G protein-coupled membrane receptor (GPCR) was captured in a defined orientation with little nonspecific binding.

Characterization of composition C4 explosives using time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy.

The application of surface analytical techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) is explored as a means of differentiating between composition C4 plastic explosives (C-4). Three different C-4 samples including U.S. military grade C-4, commercial C-4 (also from the United States), and C-4 from England (PE-4) were obtained and analyzed using both ToF-SIMS and XPS. ToF-SIMS was able to successfully discriminate between different C-4 samples with the aid of principal component analysis, a multivariate statistical analysis approach often used to reduce the dimensionality of complex data. ToF-SIMS imaging was also used to obtain information about the spatial distribution of the various additives contained within the samples. The results indicated that the samples could potentially be characterized by their 2-D chemical and morphological structure, which varied from sample to sample. XPS analysis also showed significant variation between samples, with changes in the atomic concentrations, as well as changes in the shapes of the high-resolution C 1s and O 1s spectra. These results clearly demonstrate the feasibility of utilizing both ToF-SIMS and XPS as tools for the direct characterization and differentiation of C-4 samples for forensic applications.

Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction: the role of molecular orientation, order, and coverage.

The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH(3)(CH(2))(n-1)SH, denoted by C(n), where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio R(I) for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (R(I) --> 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m(-2) to 168.3 mJ m(-2), the Young's modulus of the film E(film) decreased from 1.0 to 0.15 GPa, and the tunneling barrier height phi(0) - E(F) decreased from 2.4 to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at approximately 2.2 nm for large F. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high-resolution region scans through the film, the thickness of the oxide layer t(oxide) was found to be between 1.9 and 3.9 nm. Finally, it was shown that phi(0) - E(F) is strain-dependent, and the strain at which the film is completely displaced from under the tip is -0.17 for all values of n.

On the origins of sudden adhesion loss at a critical relative humidity: examination of bulk and interfacial contributions.

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.