On the reproducibility of electron-beam lithographic fabrication of photonic nanostructures.

Photonic nanostructures such as gratings and ring resonators have become ubiquitous building blocks in Photonics. For example, they are used in filters, they resonantly enhance signals and act as grating couplers. Much research effort is invested in using such structures to create novel functionalities, which often employs electron-beam lithography. An intrinsic issue in this field is the ability to accurately achieve a specific operating wavelength, especially for resonant systems, because nanometer-scale variations in feature size may easily detune the device. Here, we examine some of the key fabrication steps and show how to improve the reproducibility of fabricating wavelength scale photonic nanostructures. We use guided mode resonance grating sensors as our exemplar and find that the exposure condition and the development process significantly affect the consistency of the resonance wavelength, amplitude, and sensitivity of the sensor. By having careful control over these factors, we can achieve consistent performance for all the sensors studied, with less than 10% variation in their resonance behaviors. These investigations provide useful guidelines for fabricating nanostructures more reliably and to achieve a higher success rate in exploratory experiments.

Noise Tolerant Photonic Bowtie Grating Environmental Sensor.

Resonant photonic refractive index sensors have made major advances based on their high sensitivity and contact-less readout capability, which is advantageous in many areas of science and technology. A major issue for the technological implementation of such sensors is their response to external influences, such as vibrations and temperature variations; the more sensitive a sensor, the more susceptible it also becomes to external influences. Here, we introduce a novel bowtie-shaped sensor that is highly responsive to refractive index variations while compensating for temperature changes and mechanical (linear and angular) vibrations. We exemplify its capability by demonstrating the detection of salinity to a precision of 0.1%, corresponding to 2.3 × 10-4 refractive index units in the presence of temperature fluctuations and mechanical vibrations. As a second exemplar, we detected bacteria growth in a pilot industrial environment. Our results demonstrate that it is possible to translate high sensitivity resonant photonic refractive index sensors into real-world environments.

Enhancing the repeatability and sensitivity of low-cost PCB, pH-sensitive field-effect transistors.

Discrete, extended gate pH-sensitive field-effect transistors (dEGFETs) fabricated on printed circuit boards (PCBs) are a low-cost, simple to manufacture analytical technology that can be applied to a wide range of applications. Electrodeposited iridium oxide (IrOx) films have emerged as promising pH-sensitive layers owing to their theoretically high pH sensitivity and facile deposition, but typically exhibit low pH sensitivity or lack reproducibility. Moreover, to date, a combined IrOx and dEGFET PCB system has not yet been realised. In this study, we demonstrate a dEGFET pH sensor based on an extended gate manufactured on PCB that is rendered pH sensitive through an electrodeposited IrOx film, which can reliably and repeatably display beyond-Nernstian pH response. Using a combination of complementary surface analysis techniques, we show that the high pH sensitivity and repeatability of the dEGFETs are dependent on both the chemical composition and critically the uniformity of the IrOx film. The IrOx film uniformity can be enhanced through electrochemical polishing of the extended gate electrode prior to electrodeposition, leading to dEGFETs that exhibit a median pH sensitivity of 70.7 ± 5 mV/pH (n = 56) compared to only 31.3 ± 14 mV/pH (n = 31) for IrOx electrodeposited on non-polished PCB electrodes. Finally, we demonstrate the applicability of these devices by demonstrating the detection and quantification of ampicillin due to ß-Lactamase enzyme activity, thus laying the foundation for cheap and ubiquitous sensors which can be applied to a range of global challenges across healthcare and environmental monitoring.

Antibiotic-functionalized gold nanoparticles for the detection of active ß-lactamases.

Antimicrobial resistance (AMR) continues to threaten the effective treatment and prevention of bacterial infections. The spread of resistant infections is accelerated by the lack of fast and cost-effective tests for the detection of AMR at the point-of-care. We aimed to address this challenge by developing a diagnostic tool to detect one of the major forms of AMR, the ß-lactamase enzymes. Antibiotic-functionalized gold nanoparticles (AuNPs) have been successfully developed for the detection of ß-lactamases in challenging biological media, namely undiluted urine. Furthermore, these tools are compatible with samples containing a urine sample preservative (boric acid) or hematuria (blood). The functionalized AuNPs interact with the active ß-lactamases, resulting in the hydrolysis of the surface-bound antibiotics, which then inhibits binding of the AuNPs to a capture protein (a penicillin-binding protein) to indicate the presence of active ß-lactamases. We successfully integrated the antibiotic-functionalized AuNPs into a new lateral flow assay (LFA), which can be used to detect active ß-lactamases down to the detection limit of 11 nM. While we demonstrate the use of antibiotic-functionalized AuNPs in an LFA format to provide a novel method of detecting active ß-lactamases, these functionalized AuNPs are amenable to a range of alternative diagnostic technologies and could lead to vital point-of-care diagnostics for the early detection of multi-drug resistant infections.

Rational Design and Self-Assembly of Coiled-Coil Linked SasG Protein Fibrils.

Protein engineering is an attractive approach for the self-assembly of nanometer-scale architectures for a range of potential nanotechnologies. Using the versatile chemistry provided by protein folding and assembly, coupled with amino acid side-chain functionality, allows for the construction of precise molecular "protein origami" hierarchical patterned structures for a range of nanoapplications such as stand-alone enzymatic pathways and molecular machines. The Staphyloccocus aureus surface protein SasG is a rigid, rod-like structure shown to have high mechanical strength due to "clamp-like" intradomain features and a stabilizing interface between the G5 and E domains, making it an excellent building block for molecular self-assembly. Here we characterize a new two subunit system composed of the SasG rod protein genetically conjugated with de novo designed coiled-coils, resulting in the self-assembly of fibrils. Circular dichroism (CD) and quartz-crystal microbalance with dissipation (QCM-D) are used to show the specific, alternating binding between the two subunits. Furthermore, we use atomic force microscopy (AFM) to study the extent of subunit polymerization in a liquid environment, demonstrating self-assembly culminating in the formation of linear macromolecular fibrils.

Surface-Bound Antibiotic for the Detection of ß-Lactamases.

Antimicrobial resistance (AMR) has been identified as a major threat to public health worldwide. To ensure appropriate use of existing antibiotics, rapid and reliable tests of AMR are necessary. One of the most common and clinically important forms of bacterial resistance is to ß-lactam antibiotics (e.g., penicillin). This resistance is often caused by ß-lactamases, which hydrolyze ß-lactam drugs, rendering them ineffective. Current methods for detecting these enzymes require either time-consuming growth assays or antibiotic mimics such as nitrocefin. Here, we report the development of a surface-bound, clinically relevant ß-lactam drug that can be used to detect ß-lactamases and that is compatible with a range of high-sensitivity, low-cost, and label-free analytical techniques currently being developed for point-of-care-diagnostics. Furthermore, we demonstrate the use of these functionalized surfaces to selectively detect ß-lactamases in complex biological media, such as urine.