Optical Monitoring of Microplastics Filtrated from Wastewater Sludge and Suspended in Ethanol.

The abundance of microplastics (MPs) in the atmosphere, on land, and especially in water bodies is well acknowledged. In this study, we establish an optical method based on three different techniques, namely, specular reflection to probe the medium, transmission spectroscopy measurements for the detection and identification, and a speckle pattern for monitoring the sedimentation of MPs filtrated from wastewater sludge and suspended in ethanol. We used first Raman measurements to estimate the presence and types of different MPs in wastewater sludge samples. We also used microscopy to identify the shapes of the main MPs. This allowed us to create a teaching set of samples to be characterized with our optical method. With the developed method, we clearly show that MPs from common plastics, such as polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), and polyethylene (PE), are present in wastewater sludge and can be identified. Additionally, the results also indicate that the density of the plastics, which influences the sedimentation, is an essential parameter to consider in optical detection of microplastics in complex natural environments. All of the methods are in good agreement, thus validating the optics-based solution.

Towards the Development of Portable and In Situ Optical Devices for Detection of Micro and Nanoplastics in Water: A Review on the Current Status.

The prevalent nature of micro and nanoplastics (MP/NPs) on environmental pollution and health-related issues has led to the development of various methods, usually based on Fourier-transform infrared (FTIR) and Raman spectroscopies, for their detection. Unfortunately, most of the developed techniques are laboratory-based with little focus on in situ detection of MPs. In this review, we aim to give an up-to-date report on the different optical measurement methods that have been exploited in the screening of MPs isolated from their natural environments, such as water. The progress and the potential of portable optical sensors for field studies of MPs are described, including remote sensing methods. We also propose other optical methods to be considered for the development of potential in situ integrated optical devices for continuous detection of MPs and NPs. Integrated optical solutions are especially necessary for the development of robust portable and in situ optical sensors for the quantitative detection and classification of water-based MPs.

Optical transmission spectra study in visible and near-infrared spectral range for identification of rough transparent plastics in aquatic environments.

Erosion of microplastics due to residence time in aquatic environments causes roughening of the microplastic. Unfortunately, currently used measurement methods do not provide information on the surface roughness of the microplastic embedded in water. In this study we propose a novel method by using transmittance to get information on the magnitude of the surface roughness of microplastics and to rank microplastics by thickness. For such a purpose, we studied optical properties such as dispersion, absorption of both plastics and water in the partial spectral range of visible light (Vis), transmission and scattering of light by plastic sheets, as well as, the calculated sample thickness in the Vis region. These were explored for the detection of both smooth and roughened plastic sheets immersed in water. Moreover, by using the transmission spectrum and refractive index of both plastic and water it is possible to estimate the average surface roughness of plastic samples. Our results suggest that the optical properties in the Vis region offer an interesting way for the detection of both rough and smooth plastic sheets and for ranking the type of plastics in an aquatic environment.

Outlook on optical identification of micro- and nanoplastics in aquatic environments.

Plastic pollution in natural water bodies is an emerging problem that requires quick actions. Recently, the role of micro- and nanoplastics in pollution and health issues has been realized and taken seriously. In this paper, we have studied optical properties, such as NIR spectra and refractive index, of some common plastic materials and present a method and data to screen especially problematic transparent plastics with rough surface in aquatic environments. We also give an outlook of possible optical measurement methods that could be used for detection of micro- and nanoplastics.

Image Information Obtained Using a Charge-Coupled Device (CCD) Camera During an Immersion Liquid Evaporation Process for Measuring the Refractive Index of Solid Particles.

The refractive index is a fundamental physical property of a medium, which can be used for the identification and purity issues of all media. Here we describe a refractive index measurement technique to determine simultaneously the refractive index of different solid particles by monitoring the transmittance of light from a suspension using a charge-coupled device (CCD) camera. An important feature of the measurement is the liquid evaporation process for the refractive index matching of the solid particle and the immersion liquid; this was realized by using a pair of volatile and non-volatile immersion liquids. In this study, refractive indices of calcium fluoride (CaF2) and barium fluoride (BaF2) were determined using the proposed method.

Reflectometric monitoring of the dissolution process of thin polymeric films.

Pharmaceutical thin films are versatile drug-delivery platforms i.e. allowing transdermal, oral, sublingual and buccal administration. However, dissolution testing of thin films is challenging since the commonly used dissolution tests for conventional dosage forms correspond rather poorly to the physiological conditions at the site of administration. Here we introduce a traditional optical reflection method for monitoring the dissolution behavior of thin polymeric films. The substances, e.g. drug molecules, released from the film generate an increase in the refractive index in the liquid medium which can be detected by reflectance monitoring. Thin EUDRAGIT® RL PO poly(ethyl acrylate-co-methyl methacrylate-co trimethylammonioethyl methacrylate chloride) (RLPO) films containing the model drug perphenazine (PPZ) were prepared by spraying on a glass substrate. The glass substrates were placed inside the flow cell in the reflectometer which was then filled with phosphate buffer solution. Dissolution was monitored by measuring the reflectance of the buffer liquid. The method was able to detect the distinctive dissolution characteristics of different film formulations and measured relatively small drug concentrations. In conclusion, it was demonstrated that a traditional optical reflection method can provide valuable information about the dissolution characteristics of thin polymeric films in low liquid volume surroundings.

On the Immersion Liquid Evaporation Method Based on the Dynamic Sweep of Magnitude of the Refractive Index of a Binary Liquid Mixture: A Case Study on Determining Mineral Particle Light Dispersion.

This is a feasibility study of a modified immersion liquid technique for determining the refractive index of micro-sized particles. The practical challenge of the traditional liquid immersion method is to find or produce a suitable host liquid whose refractive index equals that of a solid particle. Usually, the immersion liquid method uses a set of immersion liquids with different refractive indices or continuously mixes two liquids with different refractive indices, e.g., using a pumping system. Here, the phenomenon of liquid evaporation has been utilized in defining the time-dependent refractive index variation of the host liquid. From the spectral transmittance data measured during the evaporation process, the refractive index of a solid particle in the host liquid can be determined as a function of the wavelength. The method was tested using calcium fluoride (CaF2) particles with an immersion liquid mixed from diethyl ether and diffusion pump fluid. The dispersion data obtained were consistent with the literature values thus indicating the proper functioning of the proposed procedure.

Immersion liquid techniques in solid particle characterization: A review.

Chemical, physical and optical properties of small solid particles are widely utilized in our everyday merchandises. For example, tailored particles embedded in paper or cosmetics improve the visual appearance of the products substantially. As a consequence of the small size of particles, one particle characterization tool is a microscope. It may provide e.g. the particle size, shape and the refractive index. The determination of the refractive index, using the microscope, typically exploited the so-called immersion liquid method. In this review, we provide an overview of non-imaging immersion matching techniques including immersion liquid set, the temperature, the wavelength, the double variation and the liquid evaporation methods. The basic features, benefits and limitations of each technique have been described followed by examples of potential applications in a quality monitoring of particle suspensions and colloids in industry.

Inverse Abbe-method for observing small refractive index changes in liquids.

This study concerns an optical method for the detection of minuscule refractive index changes in the liquid phase. The proposed method reverses the operation of the traditional Abbe refractometer and thus utilizes the light dispersion properties of materials, i.e. it involves the dependence of the refractive index on light wavelength. In practice, the method includes the detection of light reflection spectra in the visible spectral range. This inverse Abbe method is suitable for liquid quality studies e.g. for monitoring water purity. Tests have shown that the method reveals less than per mil NaCl or ethanol concentrations in water.

Roughened glass slides and a spectrophotometer for the detection of the wavelength-dependent refractive index of transparent liquids.

We describe a method to determine the wavelength-dependent refractive index of liquids by measurement of light transmittance with a spectrophotometer. The method is based on using roughened glass slides with different a priori known refractive indices and immersing the slides into the transparent liquid with unknown refractive index. Using the dispersion data on the glass material it is possible to find the index match between the liquid and the glass slide, and hence the refractive index of the liquid.

Assessment of wavelength dependent complex refractive index of strongly light absorbing liquids.

A practical measurement procedure for the determination of the complex refractive index of strongly absorbing liquids within a finite spectral range was developed. The method is based on separate measurements of reflectance and transmittance of the liquid sample, a property of dispersion and absorption, and exploitation of Fresnel's theory. The advantage of the method is that the knowledge of the layer thickness of the light absorbing medium, which is required typically in transmittance measurements, is not needed. In addition, both measurements, the transmittance and the reflectance, were accomplished with one spectrophotometer using a home-built reflectometer and without any sample dilution. The method is validated by numerical simulation using the Lorentz model for permittivity of an insulator, and also by experimental data obtained from three strongly absorbing offset inks, namely magenta, yellow and cyan.

Light transmission model for determination of the refractive index of solid particles in suspensions: an immersion study.

We report on a model to determine the light transmission of solid particles in immersion liquids. Using the model, optical and physical properties of suspensions such as the refractive index of the particles can be retrieved. The model can also be used in the case of colored suspensions, where the detection of only the maximum transmission would give an inaccurate refractive index for a particle. The validity of the method was tested with CaF(2) particles. Potential applications include scientific studies of the optical properties of micro- and nanoparticles or microorganisms in a liquid matrix. The model is also useful in industry where monitoring of suspensions and colloids is an issue.

Optical sensing of concentration and refractive index of pigments in a suspension.

We describe an immersion liquid and data analysis method for the simultaneous determination of the refractive index and concentration of pigments by measurement of light transmission of suspensions. A new innovation is that, in the event that two different pigments are simultaneously present in a suspension, it is possible to detect the refractive index of an unknown pigment with the aid of half-width of transmittance and, furthermore, to obtain the concentration of the unknown pigment.

Assessment of refractive index of pigments by Gaussian fitting of light backscattering data in context of the liquid immersion method.

Using a multifunction spectrophotometer, the refractive index of a pigment can be estimated by measuring the backscattering of light from the pigment in immersion liquids having slightly different refractive indices. A simple theoretical Gaussian function model related to the optical path distribution is introduced that makes it possible to describe quantitatively the backscattering signal from transparent pigments using a set of only a few immersion liquids. With the aid of the data fitting by a Gaussian function, the measurement time of the refractive index of the pigment can be reduced. The backscattering measurement technique is suggested to be useful in industrial measurement environments of pigments.

A method for the detection of the refractive index of irregular shape solid pigments in light absorbing liquid matrix.

The immersion liquid method is powerful for the measurement of the refractive index of solid particles in a liquid matrix. However, this method applies best for cases when the liquid matrix is transparent. A problem is usually how to assess the refractive index of a pigment when it is in a colored host liquid. In this article we introduce a method, and show that by combining so-called multifunction spectrophotometer, immersion liquid method and detection of light transmission and reflection we can assess the refractive index of a pigment in a colored liquid, and also the extinction or absorption coefficient of the host liquid.

Optical characterization of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA) monomers and copolymer.

OBJECTIVE: The fundamental optical characteristics of dental polymers as a function of curing time are exiguously reported in the literature. In this work the refractive index [1] and the extinction coefficient [2] of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA)-based polymers of different volume fractions have been studied as a function of the curing time. A comparison between the measured index of refraction and theoretical calculation based on molar refraction is made. Also the temporal behavior of both the refractive index and extinction coefficient is compared. METHODS: Index of refraction: BisGMA/TEGDMA monomer samples were cured with a commercial light-curing unit by using camphorquinone and DMAEMA as the photoinitiator system. Curing times of 0, 40, 60, 300, 900 and 1800 s were used. The refractive indices were determined with a refractometer at 470 nm wavelength, where the peak absorption of the camphorquinone photoinitiator is located. Extinction coefficient: BisGMA/TEGDMA samples having different lengths were cured and the relative attenuation of light at 470 nm was measured as a function of the curing time (0, 40, 60, 300, 900, 1800 s). RESULTS: The results show that in general the index of refraction increases during the photopolymerization process in a time frame from 0 to 40s and then approaches a steady-state value, which depends on the original composition of the admixture. The detailed refractive index change depends on the volume fractions of the constituent substances. The extinction coefficient decreases in a time frame from 60 to 900 s and reaches a steady-state value thereafter. SIGNIFICANCE: Knowledge of the fundamental optical characteristics of commonly used dental polymers is essential in the development of advanced light-initiated curing methods.

Estimation of effective refractive index of birefringent particles using a combination of the immersion liquid method and light scattering.

A method to detect the effective refractive index and concentration of birefringent pigments is suggested. The method is based on the utilization of the immersion liquid method and a multifunction spectrophotometer for the measurement of back scattered light. The method has applications in the measurement of the effective refractive index of pigments that are used, e.g., in the paper industry to improve the opacity of paper products.

Measurement of refractive index of isotropic particles by incorporating a multifunction spectrophotometer and immersion liquid method.

We propose a sequence of transmission and reflection measurements that makes it possible to get a relatively accurate estimate for refractive index of optically isotropic powder materials. This method makes use of immersion liquids and a multifunction spectrophotometer, which we have constructed recently. As an example we present data for CaF(2).

Complex refractive index of turbid liquids.

We present a multifunction spectrophotometer for the measurement of light reflection from thick and transmission from very thin turbid liquid samples. Moreover, we present a method to get the complex refractive index of such turbid liquids. As an example of the high performance of the device and the method, we present data on inks that are used in offset printing.

Reflectometric study of contaminant layer on a probe window.

In this study properties of a contaminant material on the surface of a probe window (prism) were investigated. The probe window, i.e., the prism, is part of the reflectometer apparatus exploited in this study. The contaminant layer was considered to originate from a pulping solution containing lignin contacting the surface of the prism. In these experiments the contact time was up to 5.0 hours. To find the values of the contaminant material properties, i.e., refractive index (n), extinction coefficient (k), and effective thickness (d), an optical model was proposed assuming that the contaminant layer is continuous and homogenous. In addition, the model includes an inter-diffusion layer existing between the contaminant layer and the lignin solution. From the fitting procedure using the optical model of depth profiling, it was found that the growth rate of the contaminant layer was approximately 60 nm per hour. The results obtained by theory and experiments were consistent for both s- and p-polarizations and for the various wavelengths of the light source (300, 400, and 500 nm).