Cellphone Camera Imaging of a Periodically Patterned Chip as a Potential Method for Point-of-Care Diagnostics

In this study, we demonstrate that a disposable chip periodically patterned with suitable ligands, an ordinary cellphone camera, and a simple pattern recognition software, can potentially be used for quantitative diagnostics. A key factor in this demonstration is the design of a calibration grid around the chip that, through a contrast transfer process, enables reliable analysis of the images collected under variable ambient lighting conditions. After exposure to a dispersion of amine terminated silica beads used as analyte mimicking pathogens, an epoxy-terminated glass substrate microcontact printed with octadecyltrichlorosilane (250 μm periodicity) developed a characteristic pattern of beads which could be easily imaged with a cellphone camera of 3.2 MP pixels. A simple pattern recognition algorithm using fast Fourier transform produced a quantitative estimate of the analyte concentration present in the test solution. In this method importantly, neither the chip fabrication process nor the fill-factor of the periodic pattern need be perfect to arrive at a conclusive diagnosis. The method suggests a viable platform that may potentially find use in fault-tolerant and robust point-of-care diagnostic applications

A Supramolecular Nanofiber-Based Passive Memory Device for Remembering Past Humidity

Memorizing the magnitude of a physical parameter such as relative humidity in a consignment may be useful for maintaining recommended conditions over a period of time. In relation to cost and energy considerations, it is important that the memorizing device works in the unpowered passive state. In this article, we report the fabrication of a humidity-responsive device that can memorize the humidity condition it had experienced while being unpowered. The device makes use of supramolecular nanofibers obtained from the self-assembly of donor–acceptor (D–A) molecules, coronene tetracarboxylate salt (CS) and dodecyl methyl viologen (DMV), respectively, from aqueous medium. The fibers, while being highly sensitive to humidity, tend to develop electrically induced disorder under constant voltage, leading to increased resistance with time. The conducting state can be regained via self-assembly by exposing the device to humidity in the absence of applied voltage, the extent of recovery depending on the magnitude of the humidity applied under no bias. This nature of the fibers has been exploited in reading the humidity memory state, which interestingly is independent of the lapsed time since the humidity exposure as well as the duration of exposure. Importantly, the device is capable of differentiating the profiles of varying humidity conditions from its memory. The device finds use in applications requiring stringent condition monitoring

Ambient Stable Tetragonal and Orthorhombic Phases in Penta-Twinned Bipyramidal Au Microcrystals

Face-centered cubic (fcc) lattice is the only known crystal structure of bulk gold. In the present work, we report the presence of body-centered tetragonal (bct) and body-centered orthorhombic (bco) phases in bipyramidal Au microcrystals with penta-twinned tips. These microcrystals have been obtained by thermolysis of (AuCl₄)⁻ stabilized with tetraoctylammonium bromide (ToABr) in air at about 220 °C for 30 min. Using a laboratory monochromatic X-ray source, the non-fcc phases could be readily detected. The remarkable occurrence of non-fcc phases of Au grown in the temperature window of 200–250 °C results from the geometrically induced strains in the bipyramids. Having derived first-principles theoretical support for the temperature-dependent stability of non-fcc Au structures under stress, we identify its origin in <i>soft</i> modes. Annealing at high temperatures relieves the stress, thus destabilizing the non-fcc phases

Transparent Pd Wire Network-Based Areal Hydrogen Sensor with Inherent Joule Heater

A high degree of transparency in devices is considered highly desirable for futuristic technology. This demands that both the active material and the electrodes are made of transparent materials. In this work, a transparent Pd wire network (∼1 cm²), fabricated using crackle lithography technique with sheet resistance and transmittance of ∼200 Ohm per square and ∼80%, respectively, serves multiple roles; besides being an electrode, it acts as an active material for H₂ sensing as well as an in-built electrothermal heater. The sensor works over a wide range of hydrogen (H₂) concentration down to 0.02% with a response time of ∼41 s, which could be improved to ∼13 s by in situ Joule heating to ∼75 °C. Importantly, the device has the potential of scale-up to a window size transparent panel and to be flexible when desired

Intrinsic Nature of Graphene Revealed in Temperature-Dependent Transport of Twisted Multilayer Graphene

Graphene in its purest form is expected to exhibit a semiconducting to metallic transition in its temperature-dependent conductivity as a result of the interplay between Coulomb disorder and phonon scattering, the transition temperature, <i>T</i>_c, depending sensitively on the disorder induced carrier density (<i>n</i>_c). Even for good quality graphene, the <i>n</i>_c can be quite high (∼10¹² cm^–2) and the transition temperature may be placed well above the ambient, practically rendering it to be only semiconducting over a wide range of temperature. Here we report an experimental study on the transport behavior of twisted multilayer graphene (tMLG) exhibiting <i>T</i>_c well below the ambient temperature. The graphene layers in these tMLG are highly decoupled with one another due to the angular rotation among them; as a result, they exhibit very high Raman I_2D/I_G values (up to 12) with narrow 2D width (16–24 cm^–1). The observed <i>T</i>_c values seem to go hand in hand with the Raman I_2D/I_G values; a multilayer with mean I_2D/I_G value of 4.6 showed a <i>T</i>_c of 180 K, while that with mean I_2D/I_G of 4.9 showed lower a <i>T</i>_c of 160 K. Further, another multilayer with even higher mean I_2D/I_G value of 6.9 was metallic down to 5 K, indicating a very low disorder. The photoresponse behavior also corroborates well with the transition in transport behavior

Highly Decoupled Graphene Multilayers: Turbostraticity at its Best

The extraordinary properties of graphene are truly observable when it is suspended, being free from any substrate influence. Here, a new type of multilayer graphene is reported wherein each layer is turbostratically decoupled, resembling suspended graphene in nature, while maintaining high degree of 2D crystallinity. Such defect-free graphene multilayers have been made over large areas by Joule heating of a Ni foil coated with a solid hydrocarbon. Raman spectra measured on thick flakes of turbostratically single layer graphene (T-SLG) (100–250 nm) have shown characteristics similar to suspended graphene with very narrow 2D bands (∼16 cm^–1) and <i>I</i>_2D/<i>I</i>_G ratios up to 7.4, importantly with no D band intensity. Electron diffraction patterns showed sets of diffraction spots spread out with definite angular spacings, reminiscent of the angular deviations from the AB packing which are responsible for keeping the layers decoupled. The <i>d</i>-spacing derived from X-ray diffraction was larger (by ∼0.04 Å) compared to that in graphite. Accordingly, the <i>c</i>-axis resistance values were three orders higher, suggesting that the layers are indeed electronically decoupled. The high 2D crystallinity observed along with the decoupled nature should accredit the observed graphene species as a close cousin of suspended graphene

Spray Coating of Crack Templates for the Fabrication of Transparent Conductors and Heaters on Flat and Curved Surfaces

Transparent conducting electrodes (TCEs) have been made on flat, flexible, and curved surfaces, following a crack template method in which a desired surface was uniformly spray-coated with a crackle precursor (CP) and metal (Ag) was deposited by vacuum evaporation. An acrylic resin (CP1) and a SiO₂ nanoparticle-based dispersion (CP2) derived from commercial products served as CPs to produce U-shaped cracks in highly interconnected networks. The crack width and the density could be controlled by varying the spray conditions, resulting in varying template thicknesses. By depositing Ag in the crack regions of the templates, we have successfully produced Ag wire network TCEs on flat-flexible PET sheets, cylindrical glass tube, flask and lens surface with transmittance up to 86%, sheet resistance below 11 Ω/□ for electrothermal application. When used as a transparent heater by joule heating of the Ag network, AgCP1 and AgCP2 on PET showed high thermal resistance values of 515 and 409 °C cm²/W, respectively, with fast response (<20 s), requiring only low voltages (<5 V) to achieve uniform temperatures of ∼100 °C across large areas. Similar was the performance of the transparent heater on curved glass surfaces. Spray coating in the context of crack template is a powerful method for producing transparent heaters, which is shown for the first time in this work. AgCP1 with an invisible wire network is suited for use in proximity while AgCP2 wire network is ideal for use in large area displays viewed from a distance. Both exhibited excellent defrosting performance, even at cryogenic temperatures

Microscopic Evaluation of Electrical and Thermal Conduction in Random Metal Wire Networks

Ideally, transparent heaters exhibit uniform temperature, fast response time, high achievable temperatures, low operating voltage, stability across a range of temperatures, and high optical transmittance. For metal network heaters, unlike for uniform thin-film heaters, all of these parameters are directly or indirectly related to the network geometry. In the past, at equilibrium, the temperature distributions within metal networks have primarily been studied using either a physical temperature probe or direct infrared (IR) thermography, but there are limits to the spatial resolution of these cameras and probes, and thus, only average regional temperatures have typically been measured. However, knowledge of local temperatures within the network with a very high spatial resolution is required for ensuring a safe and stable operation. Here, we examine the thermal properties of random metal network thin-film heaters fabricated from crack templates using high-resolution IR microscopy. Importantly, the heaters achieve predominantly uniform temperatures throughout the substrate despite the random crack network structure (e.g., unequal sized polygons created by metal wires), but the temperatures of the wires in the network are observed to be significantly higher than the substrate because of the significant thermal contact resistance at the interface between the metal and the substrate. Last, the electrical breakdown mechanisms within the network are examined through transient IR imaging. In addition to experimental measurements of temperatures, an analytical model of the thermal properties of the network is developed in terms of geometrical parameters and material properties, providing insights into key design rules for such transparent heaters. Beyond this work, the methods and the understanding developed here extend to other network-based heaters and conducting films, including those that are not transparent

Flexible Palladium-Based H₂ Sensor with Fast Response and Low Leakage Detection by Nanoimprint Lithography

Flexible palladium-based H₂ sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H₂ sensor that is capable of detecting H₂ gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H₂ sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H₂ sensors with improved durability, sensitivity, and response time in comparison to palladium thin films. At ambient pressure and temperature, the device showed a fast response time of 18 s at a H₂ concentration of 3500 ppm. At 50 ppm concentration, the response time was found to be 57 s. The flexibility of the sensor does not appear to compromise its performance

Extraordinarily Stable Noncubic Structures of Au: A High-Pressure and -Temperature Study

Although the stability of Au in the face-centered cubic (FCC) phase at high temperatures and pressures has been well studied, the stability in other lattice phases rarely encountered in crystallite domains in microscopy studies has not been explored much because of their nanometric extensions. A recent report on Au microcrystallites crystallized in body-centered tetragonal (BCT) and body-centered orthorhombic (BCO) phases prompted the work presented here, in which we have investigated for the first time the structural stability of the BCT and BCO phases at high temperatures and separately at high pressures using high-energy synchrotron X-ray diffraction. A reversible phase transition was observed for pressures of up to ∼40 GPa, indicating unusual stability of the non-FCC Au phases. However, during a high-temperature treatment at ∼700 °C, the transformation to FCC was irreversible