The correct detection and quantification of pollutants in water is key to regulating their presence in the environment. Biosensors offer several advantages, such as minimal sample preparation, short measurement times, high specificity and sensibility and low detection limits. The purpose of this review is to explore the different types of optical biosensors, focusing on their biological elements and their principle of operation, as well as recent applications in the detection of pollutants in water. According to our literature review, 33% of the publications used fluorescence-based biosensors, followed by surface plasmon resonance (SPR) with 28%. So far, SPR biosensors have achieved the best results in terms of detection limits. Although less common (22%), interferometers and resonators (4%) are also highly promising due to the low detection limits that can be reached using these techniques. In terms of biological recognition elements, 43% of the published works focused on antibodies due to their high affinity and stability, although they could be replaced with molecularly imprinted polymers. This review offers a unique compilation of the most recent work in the specific area of optical biosensing for water monitoring, focusing on both the biological element and the transducer used, as well as the type of target contaminant. Recent technological advances are discussed.

This paper addresses a long-standing concern of whether one can measure the refractive index of a turbid colloid as if it were an equivalent homogeneous medium using standard techniques. We analyze the accuracy of determining the effective refractive index of turbid colloids when measuring the critical angle by reflection of light in an internal reflection configuration and then using Snell’s law. It is assumed that the critical angle is taken as the inflection point of the reflectance curve and that the effective refractive index of the turbid colloid is well approximated by the van de Hulst approximation. Experimental measurements are used to show the viability of measuring the inflection point of an angular-reflectance curve with highly turbid colloids. The refractive index of aqueous suspensions of T i O 2 and latex particles (sizes in the range of 100–500 nm) were determined with an accuracy of ±0.0025. Then, a multiple-scattering model for the coherent reflection of light from turbid colloids is used to produce 2D maps of the estimated error in determining the effective refractive index of turbid colloids by the critical angle method as a function of the size and volume-filling fraction of the colloidal particles.

The growing interest in functional transition metal oxides for efficient energy consumption or in the bio-sensing process; indicates that is necessary to develop a new theoretical method that describes experiments. This article presents a new theoretical methodology to characterize molybdenum trioxide (MoO₃) thin films doped with resonant gold – nanoparticles (Au – NPs) and non-resonant titanium – nanoparticles (Ti – NPs). The modulation of surface plasmon resonance (SPR) and the implications in the MoO₃ transmittance spectrum is described by applying an effective medium theory. The transmittance modulation was modified by variating three parameters, the radius of the NPs, the concentration of the NPs as well as the variation of the MoO₃ thin films thickness. It was found that the nanoparticles concentration is the most important parameter in the transmittance modulation. Additionally, the orthorhombic and monoclinic structure of MoO₃ was studied, from which it was obtained that the monoclinic structure of the MoO₃ doped with Au – NPs favors the reduction in the transmittance values in the visible region which is associated with the increase of the SPR signal. Similar analyses are performed for non-resonant nanoparticles such as Ti, where it was found that optical modulation is not as marked as the case of gold nanoparticles.

We calculated the real and imaginary parts of the effective refractive index of blood as functions of wavelength from 400 to 800 nm; we employed van de Hulst's theory, together with the anomalous diffraction approximation, for the calculation. We modelled blood as a mixture of plasma and erythrocytes. Our results indicate that erythrocyte orientation has a strong effect on , making blood an optically anisotropic medium except when the erythrocytes are randomly oriented. In the case in which their symmetry axis is perpendicular to the wave vector, equals the refractive index of plasma at certain wavelengths. Furthermore, the erythrocytes' shape affects their contribution to in an important way, implying that studies on the effective refractive index of blood should avoid approximating them as spheres or spheroids. Finally, the effective refractive index of blood predicted by van de Hulst's theory is different from what would be obtained by averaging the refractive indices of its constituents weighted by volume; such a volume-weighted average is appropriate only for haemolysed blood. We then measured the real part of the refractive index of various blood solutions using two different experimental setups. One of the most important results of our expriment is that is measurable to a good degree of precision even for undiluted blood, although not all measuring apparatuses are appropriate. The experimental data is self-consistent and in reasonable agreement with our theoretical calculations.