Špela Korent Urek

Optical Chemical Sensors:Design and Applications

Optical sensors, or opt(r)odes, represent a group of chemical sensors in which electromagnetic (EM) radiation is used to generate the analytical signal in a transduction element. The interaction of this radiation with the sample is evaluated from the change of a particular optical parameter and is related to the concentration of the analyte (Blum, 1997). Typically, an optical chemical sensor consists of a chemical recognition phase (sensing element or receptor) coupled with a transduction element (Fig. 1). The receptor identifies a parameter, e.g., the concentration of a given compound, pH, etc., and provides an optical signal proportional to the magnitude of this parameter. The function of the receptor is fulfilled in many cases by a thin layer that is able to interact with the analyte molecules, catalyse a reaction selectively, or participate in a chemical equilibrium together with the analyte. The transducer translates the optical signal produced by the receptor into a measurable signal that is suitable for processing by amplification, filtering, recording, display, etc. (Grundler, 2007; Nagl & Wolfbeis, 2008).

Nano-Based Optical Chemical Sensors

The development of nanoscale materials for optical chemical sensing applications has emerged as one of the most important research areas of interest over the past decades. Nanomaterials exhibit highly tunable size- and shape-dependent chemical and physical properties, show unique surface chemistry, thermal and electrical properties, high surface area and large pore volume per mass unit area. Because of their unique and advantageous features they can help to improve sensitivity, response time and detection limit of sensors. In this review, recently developed photoluminescence-based optical chemical nanosensors are presented. Some future trends of the nanomaterial-based optical chemical sensors are given.

Nanostructured Materials Use in Sensors: Their Benefits and Drawbacks

The development of nanoscale materials for optical chemical sensing applications has emerged as one of the most important research areas of interest over the past decades. In this chapter we firstly present some general aspects of nanostructured materials and give a description on the analytical aspects of sensors and sensing principles. The broad variety of nanomaterials as well as sensors’ design made us to limit our presentation, which concentrates on nanomaterials, such as quantum dots, polymer- and sol-gel-based particles. The benefits and drawbacks of the properties of these nanomaterials used in optical sensing applications are given, and the recently developed optical chemical sensors and probes based on photoluminescence are overviewed. Finally, some future trends of the nanomaterial-based optical chemical sensors are given.

Sol-gel based optical chemical sensors

The growing activity in the field of optical chemical sensors has resulted in numerous sensing schemes, new indicator dyes, various polymeric matrix, size and shapes and highly diversified methods of immobilization. The sensor characteristics are dependent upon the choice of indicator, polymer, immobilization technique, and also size. Sol-gel technology provides a low-temperature method for obtaining porous silicate glass matrices. It enables to obtain material in the form of films, powders, monoliths, fibres or nanoparticles. Organic reagents and molecular receptors can be easily immobilized in the matrices. Moreover, one of the unique features of the sol-gel process is that the properties of the final network structure, such as hydrophobicity, thickness, porosity, flexibility, reactivity and stability can be easily tailored by controlling the process conditions, the type and the size of the precursors and catalysis. Here we will report about several sensor designed over the years based on sol-gel materials for monitoring and controlling different parameters, such as heavy metals, amines, phosphates, organophosphates.

Fluorescence-based optical chemical sensors for personal protection

The World Health Organization has reported that each year approximately 3 million people are poised by organophosphate substances (pesticides and nerve agents) resulting in 220,000 deaths. Organophosphates (OP) are toxic compounds which cause rapid and severe inhibition of serine proteases, most markedly acetylcholinesterase, which is vital to nerve function. This inhibition is often fatal. OP nerve agents are generally stable, easy to disperse, and highly toxic. They can be absorbed through the skin, by ingestion, or by respiration. A release of a nerve agent has the potential to rapidly affect a large number of people. The ease of manufacturing and dispensability of nerve agents, as well as available, inexpensive starting materials make these agents a weapon of choice for criminal terrorist attacks. One of the major steps toward protection against dangerous substances is to develop sensor devices that can act as an early warning system to the endangered people.

Design and Investigation of Optical Properties of N-(Rhodamine-B)-Lactam-Ethylenediamine (RhB-EDA) Fluorescent Probe

This study presents chemical modification of a Rhodamine B (RhB) sensor probe by ethylenediamine (EDA), and investigation of its spectral as well as sensor properties to the various metals. The synthesised N-(Rhodamine-B)-lactam-ethylenediamine (RhB-EDA) fluorescent probe shows interesting optical sensor properties, and high sensitivity and selectivity to Ag+ ions among all the tested metal ions (K+, Mg2+, Cu2+, Ni2+, Fe2+, Pb2+, Na+, Mn2+, Li+, Al3+, Co2+, Hg2+, Sr2+, Ca2+, Ag+, Cd2+ and Zn2+), while the well-known Rhodamine B (RhB) fluorescent probe shows much less sensitivity to Ag+ ions, but high sensitivity to Fe2+ ions. The novel fluorescent sensor probe RhB-EDA has the capabilities to sense Ag+ ions up to µM ranges by using the fluorescence quenching approach. The probe displayed a dynamic response to Ag+ in the range of 0.43 × 10−3–10−6 M with a detection limit of 0.1 μM. The sensing system of an RhB-EDA novel fluorescent probe was optimised according to the spectral properties, effect of pH and buffer, photostability, incubation time, sensitivity, and selectivity. Since all the spectral and sensing properties were tested in green aqueous media, although many other similar sensor systems rely on organic solvent solutions, the RhB-EDA sensing probe may be a good candidate for measuring Ag+ ions in real-life applications.

Magnetic Silica Nanocomposites as Optical Tools in Biomedical Applications

As a result of their size and versatile chemistry, today’s nanomaterials represent powerful tools for several biomedical applications. Various types of nanomaterials have proven to be practical, not only for determining clinically relevant parameters, but also for diagnostics, drug delivery and the treatment of diseases (e.g., cancer). Of particular promise are those nanocomposite structures with multifunctional capabilities. In this chapter we focus on magnetic silica nanocomposites, combined with an optical component, such as organic fluorescent dyes or quantum dots. The most important characteristics of these nanomaterials are presented, together with their specific uses in biomedical applications. It was observed that findings based on in-vitro measurements were not always in agreement with in-vivo applications. Additionally, the use of these nanocomposites in clinical trials remains a long-term goal.

Quantum Dots Based Optical Sensors

Luminescent sensors are chemical systems that can deliver information on the presence of selected analytes through the variations in their luminescence emission. With the advent of luminescent nanoparticles several new applications in the field of chemical sensing were explored. Among them, quantum dots (QD) represent inorganic semiconductor nanocrystals that are advantageous over conventional organic dyes from many different points of view. In this short review, the optical detection of various analytes using QD-based probes/sensors is presented and significant sensors characteristics are discussed. The biosensing approaches are not included in this article.

Fluorescent-based chemical sensor for organophosphate detection

We present a new optical sensor for the detection of organophosphates by incorporating fluorescent indicator dye into sol-gel material. We used different configurations of immobilization matrices such as thin film and spherical nanoparticles. The sensor thin films were prepared by using acid-catalyzed sol-gel process and the spherical nanoparticles by modified Stöber method. The effects of configuration matrices on the sensors characteristic were studied. The use of dye-doped nanoparticles improved the detection limit from 0.69 μM to 17 nM, response time from 600 s to 12 s, precision and sensitivity, but reduced the sensors working rage from 6.9×10-7 M - 6.9×10-3 M to 1.75×10-8M - 2.3×10-7 M.