Machine Learning Models for Enhanced Estimation of Soil Moisture Using Wideband Radar Sensor.

In this paper, machine learning models for an effective estimation of soil moisture content using a microwave short-range and wideband radar sensor are proposed. The soil moisture is measured as the volumetric water content using a short-range off-the-shelf radar sensor operating at 3-10 GHz. The radar captures the reflected signals that are post processed to determine the soil moisture which is mapped to the input features extracted from the reflected signals for the training of the machine learning models. In addition, the results are compared and analyzed with a contact-based Vernier soil sensor. Different machine learning models trained using neural network, support vector machine, linear regression and k-nearest neighbor are evaluated and presented in this work. The efficiency of the model is computed using root mean square error, co-efficient of determination and mean absolute error. The RMSE and MAE values of KNN, SVM and Linear Regression are 11.51 and 9.27, 15.20 and 12.74, 3.94 and 3.54, respectively. It is observed that the neural network gives the best results with an R2 value of 0.9894. This research work has been carried out with an intention to develop cost-effective solutions for common users such as agriculturists to monitor the soil moisture conditions with improved accuracy.

A portable SERS sensor for pyocyanin detection in simulated wound fluid and through swab sampling.

A portable surface-enhanced Raman spectroscopy (SERS) sensor for detecting pyocyanin (PYO) in simulated wound fluid and from bacteria samples was developed. Solution-phase SERS detection protocols are designed to be compatible with two different clinical practices for wound exudate collection, namely negative pressure liquid collection and swabbing. For citrate-coated metal nanoparticles of three different compositions, i.e. gold (AuNPs), alloyed silver/gold (AgAuNPs), and silver (AgNPs), we firstly confirmed their interaction with PYO in the complex wound fluid, using fluorescence quenching experiments, which rationalized the Raman enhancement effects. We then demonstrated the Raman enhancement effects of the metal nanoparticles in the order of AgNPs > AgAuNPs > AuNPs. The limit of detection (LOD) achieved for PYO is 1.1 µM (in a linear range of 0.1-25 µM by the AgNPs), 10.9 µM (in a linear range of 5-100 µM, by the AgAuNPs), and 17.7 µM (in a linear range of 10-100 µM by the AuNPs). The AgNP and AgAuNP sensors together cover the sensitivity and dynamic range requirements for the clinical detection of wound infection, where PYO is present at a concentration of 1-50 µM. In addition, sterilized cotton swabs were used to collect wound fluid and transfer samples into AgNP solution for SERS measurements. This detection protocol was completed within 5 minutes with a LOD of 23.1 µM (in a linear range of 15-100 µM). The SERS sensing protocol was validated by its successful detection of PYO in cultured Pseudomonas aeruginosa bacteria. The findings presented in this work pave the way towards point-of-care diagnostics of wound infections.

Study of the Effect of Anisotropic Gold Nanoparticles on Plasmonic Coupling with a Photosensitizer for Antimicrobial Film.

Development of antimicrobial surfaces for sterilization is much needed to avoid the spreading of drug resistant bacteria. Light can activate antimicrobial surfaces by an interaction between nanoparticles and a photosensitizer dye to produce a steady and efficient killing of bacteria. The film studied in this work contains gold nanorods (AuNRs) of 32 nm length and 16 nm diameter and gold nanostars (AuNSs) of 50 nm of diameter, in combination with crystal violet (CV) dye. The surface plasmon resonance (SPR) of the nanoparticles used in the film was mathematically simulated and characterized to understand different SPR between the particles. Their effects on plasmonic coupling with the dye, and thus the production of reactive oxygen species (ROS) and consequently the activity of the film against bacteria, were studied. The films showed great antimicrobial activity against Gram-negative bacteria (E. coli) in 4 h of light exposure; when modified with AuNSs, it could kill E. coli with 5 orders of magnitude (5-log), and the one modified with AuNRs could kill with 4 order of magnitude (4-log), while maintaining partial activity against Gram-positive bacteria (S. aureus), i.e. being able to kill with 2.5 orders of magnitude by the film containing AuNSs and 3 orders of magnitude by those containing AuNRs. The differential response of Gram-negative and Gram-positive bacteria to the ROS generated by the films would allow a more targeted approach for specific bacterial species, for example, surfaces of bedpans or common contact surfaces (handles, handrails, etc.) that are contaminated principally by Gram-negative or Gram-positive bacteria, respectively.

Maskless fabrication of slanted annular aperture arrays.

We report maskless fabrication of high-aspect-ratio slanted annular aperture arrays (SAAAs) in gold films using focused ion beam lithography. By tilting the substrate, SAAAs with the desired tilting angle can be fabricated. Our experimental results demonstrate accurate control over aperture size, obliqueness, and reproducibility. We also show that the resulted plasmonic resonances of SAAAs can be effectively tuned via obliqueness control. This versatile approach may enable fabrication of more complicated plasmonic nanostructures. The demonstrated gold SAAAs could also find many potential applications in plasmon-assisted sensing and surface enhanced spectroscopy.

Multiscale ommatidial arrays with broadband and omnidirectional antireflection and antifogging properties by sacrificial layer mediated nanoimprinting.

Moth's eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

Optical properties of ultrafine line and space polymeric nanogratings coated with metal and metal-dielectric-metal thin films.

Noble metal and metal-dielectric-metal ultrathin films were deposited on the surfaces of ultrafine polymeric nanogratings, which were fabricated using nanoimprint lithography. Experimental results showed dramatic differences of the surface morphologies for single metal and triple metal-dielectric-metal films deposited on flat and corrugated polymeric surfaces. The effect of the surface morphology on the optical properties was hence investigated and analyzed under linearly polarized light. The surface plasmon resonances of single metal and triple metal-dielectric-metal films deposited on polymeric nanograting surfaces were also characterized based on the Kretschmann prism-coupling method. The single metal and triple metal-dielectric-metal films deposited on polymeric nanograting surfaces are important for the study of photon-plasmon interactions (i.e. couplings and conversions) at the interfaces between a nanograting and metal films.

Plasmonic wave plate based on subwavelength nanoslits.

We propose a quarter-wave plate based on nanoslits and analyze it using a semianalytical theory and simulations. The device comprises two nanoslits arranged perpendicular to one another where the phases of the fields transmitted by the nanoslits differ by λ/4. In this way, the polarization state of the incident light can be changed from linear to circular or vice versa. The plasmonic nanoslit wave plate is thin and has a subwavelength lateral extent. We show that the predictions for the phase shift obtained from a semianalytical model are in very good agreement with simulations by the finite difference time domain method.