The field of chemical sensing is becoming ever more dependent upon novel materials. Polymers, crystals, glasses, particles, and nanostructures have made a profound impact and have endowed modern sensory systems with superior performance. Electronic polymers have emerged as one of the most important classes of transduction materials; they readily transform a chemical signal into an easily measured electrical or optical event. Although our group reviewed this field in 2000,1 the high levels of activity and the impact of these methods now justify a subsequent review as part of this special issue. In this review we restrict our discussions to purely fluorescence-based methods using conjugated polymers (CPs). We further confine our detailed coverage to articles published since our previous review and will only detail earlier research in this Introduction to illustrate fundamental concepts and terminology that underpin the recent literature.

Chemical sensors based on amplifying fluorescent conjugated polymers.

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

SERS: Materials, applications, and the future

Nondestructive measurement of fruit and vegetable quality by means of NIR spectroscopy: A review

https://www.uta.edu/rfmems/GI_Sensor/papers/ac040049d.pdf

Fiber-Optic Chemical Sensors and Biosensors

We have designed and demonstrated a standoff Raman system for detecting high explosive materials at distances up to 50 meters in ambient light conditions. In the system, light is collected using an 8-in. Schmidt-Cassegrain telescope fiber-coupled to an f/1.8 spectrograph with a gated intensified charge-coupled device (ICCD) detector. A frequency-doubled Nd : YAG (532 nm) pulsed (10 Hz) laser is used as the excitation source for measuring remote spectra of samples containing up to 8% explosive materials. The explosives RDX, TNT, and PETN as well as nitrate- and chlorate-containing materials were used to evaluate the performance of the system with samples placed at distances of 27 and 50 meters. Laser power studies were performed to determine the effects of laser heating and photodegradation on the samples. Raman signal levels were found to increase linearly with increasing laser energy up to approximately 3 x 10(6) W/cm2 for all samples except TNT, which showed some evidence of photo- or thermal degradation at higher laser power densities. Detector gate width studies showed that Raman spectra could be acquired in high levels of ambient light using a 10 microsecond gate width.

Standoff detection of high explosive materials at 50 meters in ambient light conditions using a small Raman instrument.

Remote analysis for challenging applications may be possible because LIBS uses only light and collects only photons.

https://pubs.acs.org/doi/pdf/10.1021/ac069342z

Emission enhancement mechanisms in dual-pulse LIBS.

In this paper we report the first observations of dual-pulse laser-induced breakdown spectroscopy (LIBS) signal enhancements by using a pre-ablation spark. In this technique a laser pulse is brought in parallel to the sample surface and focused a few millimeters above it to form an air plasma or air spark. A few microseconds later a second laser pulse, which is focused on the sample, ablates sample material and forms the LIBS plasma from which analyte emission occurs. In this way, large LIBS signal enhancements, 11- to 33-fold, are observed for copper and lead, respectively, relative to the signal in the absence of the air spark. In all cases where enhanced LIBS signals are seen, greatly enhanced sample ablation also occurs.

Dual-Pulse LIBS Using a Pre-Ablation Spark for Enhanced Ablation and Emission

In this paper, we investigate the effect of dual-pulse timing on material ablation, plasma temperature, and plasma size for pre-ablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS) Although the plasma temperature increases for dual-pulse excitation, the signal enhancement is most easily attributed to increased sample ablation Plasma images show that the magnitude of the enhancement can be affected by the collection optic and by the collection geometry Enhancements calculated using the total integrated intensity of the plasma are comparable to those measured using fiber-optic collection

Effect of Pulse Delay Time on a Pre-Ablation Dual-Pulse LIBS Plasma:

Nanosecond and femtosecond laser pulses were combined in an orthogonal preablation spark dual-pulse laser-induced breakdown spectroscopy (LIBS) configuration. Even without full optimization of interpulse alignment, ablation focus, large signal, signal-to-noise ratio, and signal-to-background ratio enhancements were observed for both copper and aluminum targets. Despite the preliminary nature of this study, these results have significant implications in the attempt to explain the sources of dual-pulse LIBS enhancements.

S. Michael Angel

Papers

Standoff detection of high explosive materials at 50 meters in ambient light conditions using a small Raman instrument.

Emission enhancement mechanisms in dual-pulse LIBS.

Dual-Pulse LIBS Using a Pre-Ablation Spark for Enhanced Ablation and Emission

Effect of Pulse Delay Time on a Pre-Ablation Dual-Pulse LIBS Plasma:

Dual-pulse laser-induced breakdown spectroscopy with combinations of femtosecond and nanosecond laser pulses