Speciation Analysis of Metals and Metalloids by Surface Enhanced Raman Spectroscopy

The presence of metalloids and heavy metals in the environment is of critical concern due to their toxicological impacts. However, not all metallic species have the same risk level. Specifically, the physical, chemical, and isotopic speciation of the metal(loids) dictate their metabolism, toxicity, and environmental fate. As such, speciation analysis is critical for environmental monitoring and risk assessment. In the past two decades, surface-enhanced Raman spectroscopy (SERS) has seen significant developments regarding trace metal(loid) sensing due to its ultrahigh sensitivity, readiness for in situ real-time applications, and cost-effectiveness. However, the speciation of metal(loid)s has not been accounted for in the design and application of SERS sensors. In this Perspective, we examine the potential of SERS for metal(loid) speciation analysis and highlight the advantages, progress, opportunities, and challenges of this application.

Raman mapping for determination of TiO2 in different solid food samples by multivariate curve resolution with alternating least squares

Titanium dioxide is a food additive commonly used as a white food coloring (E171). Its wide use by the food industry associated with the nanometric size distribution of the particles of this pigment has shown high genotoxicity associated with recurrent exposure by ingestion. Therefore, the use of E171 in food products has already been banned by some industries and in the European Union. Such banishment should soon be extended to other countries around the world, making it important to establish techniques for the efficient determination of TiO₂ in different food products. The association between hyperspectral images and chemometric tools can be useful in this sense, aiming to enable the use of a single method for sample preparation and analysis of different types of food. Thus, the present work aims to evaluate the use of Raman mapping associated with the resolution of multivariate curves with alternating least squares (MCR-ALS) for the determination of titanium dioxide in solid food samples with different compositions, without the need to introduce specific sample preparation. The proposed method allowed for the first-time quantification of TiO₂ in different food matrices without specific sample preparation, with a simple, rapid, accurate (93% of recovery), low detection limits (0.0111% m/m) and quantification (0.0370% m/m) and adequate linearity (r = 0.9990) and precise (standard deviation around 0.020-0.030% w/w) methodology. Such results highlight the potential use of Raman mapping associated with the MCR-ALS for quantification of the nano-TiO₂ in commercial samples.

TiO2 nanoparticles negatively impact the bioavailability and antioxidant activity of tea polyphenols

This study was to investigate the influence of TiO₂ nanoparticles (NPs) on the stability, bioavailability, and antioxidant activity of co-ingested tea polyphenols extract using an in vitro digestion model. The tea polyphenol contents decreased significantly after addition of 0.5 % (w/w) TiO₂ NPs. The gallocatechin gallate level decreased the most, changing from 101.9 to 27.2 µg/mL (about 73.3%). The TiO₂ NPs also reduced the bioavailability of the tea polyphenols in a dose-dependent manner, which was ascribed to the formation of large polyphenol-TiO₂ NP complex aggregates that could not pass through the pores in the dialysis tube used to simulate the gut wall. Additionally, the TiO₂ NPs decreased the antioxidant activity of the tea polyphenols within the simulated gastrointestinal tract. In summary, our results show that high levels of TiO₂ NPs (but within the current legal limits in many countries) may negatively impact the bioavailability and bioactivity of polyphenols in foods.

A new nanometrological strategy for titanium dioxide nanoparticles screening and confirmation in personal care products by CE-spICP-MS

A new nanometrological approach was developed for screening of titania nanoparticles by capillary electrophoresis after adsorption of a target analyte namely l-cysteine onto the nanoparticles in a sodium phosphate buffer, followed by titanium elemental analysis by means of inductively-coupled plasma-mass spectrometry and size distribution measurements by single-particle mode. This analytical strategy involved a first screening of nanotitania in actual samples by electrophoresis, sensitivity being enhanced by cysteine which acts as a nanoparticles stabiliser. Detection and quantitation limits were 0.31 ng μL^-1 and 1.03 ng μL^-1 respectively for anatase nanoparticles in capillary electrophoresis, and a high amount of titanium was found in the samples subject to study (lip balm and two types of toothpaste) by total elemental analysis. Besides, the potential of single-particle modality for inductively-coupled plasma-mass spectrometry was exploited for a verification of particle size distribution, then confirming the presence of titanium dioxide nanoparticles as an ingredient in the composition of the real samples and validating the overall strategy herein presented.

Phosphoric acid functionalized magnetic sorbents for selective enrichment of TiO2 nanoparticles in surface water followed by inductively coupled plasma mass spectrometry detection

Phosphoric acid functionalized superparamagnetic iron oxide was synthesized, and different adsorption behavior of TiO₂ NPs and titanium ions on it was found. By means of dispersion-corrected density functional theory (DFT-D), the adsorption mechanism of TiO₂ NPs and titanium ions on the functionalized sorbents was explored, and the difference in the adsorption behavior was attributed to the different deprotonated forms of phosphates and the competitive adsorption of OH^- anion with respect to either TiO₂ NPs or aqueous titanium ions. Based on the different adsorption performance of phosphoric acid functionalized sorbents for TiO₂ NPs and titanium ions under pH 3, a method by combining magnetic solid phase extraction (MSPE) with inductively coupled plasma mass spectrometry (ICP-MS) was established for the selective quantification of trace TiO₂ NPs in environmental water. Under the optimal experimental conditions, the detection limit of TiO₂ NPs was 17 ng/L with an enrichment factor of 400. The developed MSPE-ICPMS method was applied to the detection of trace TiO₂ NPs in the Yangtze River and the East Lake water. Sub μg/L level of TiO₂ NPs was found in the tested water samples, and recoveries of 91-110% and 90-110% were obtained for TiO₂ NPs at three concentration levels in spiked water samples, respectively. The developed method exhibited high adsorption capacity and low detection limit for target TiO₂ NPs, and was demonstrated with great potential for monitoring TiO₂ NPs in the environment.

Titanium Dioxide in Food Products: Quantitative Analysis Using ICP-MS and Raman Spectroscopy

Titanium dioxide (TiO₂) is commonly used as a color additive in food products. In this study, a total of 11 food products, such as a coffee cream, yogurt snack, hard candy, and chewy candy, that are widely consumed by adults or children were investigated. For characterization of particle size, size distribution, crystallinity, and concentration of TiO₂, particles were first extracted using an acid digestion method from food, and various analytical techniques were applied. All products investigated in this study contained nanosized TiO₂ particles (21.3-53.7%) in the anatase phase. The particle size of TiO₂ was in the range of 26.9-463.2 nm. The concentration of TiO₂ in the products ranged from 0.015% (150 ppm) to 0.462% (4620 ppm). These values obtained using inductively coupled plasma-mass spectrometry (ICP-MS) were considered as the reference and were compared with Raman results to evaluate the feasibility of using the Raman method to quantitate TiO₂ in food products. The Raman method developed in this study proved to effectively analyze anatase TiO₂ in food products at levels of several hundred parts per million or greater. Limitations of using the Raman method as a quick screening tool for determination of TiO₂ are also discussed.

Review of analytical studies on TiO2 nanoparticles and particle aggregation, coagulation, flocculation, sedimentation, stabilization

Titanium dioxide (TiO₂) nanoparticles (NPs) have been widely used in industrial and consumer products. Comprehensive and accurate detection, characterization, and quantification of TiO₂ NPs are important for understanding the specific property, behavior, fate, and potential risk of TiO₂ NPs in natural and engineered environments. This review provides a summary of recent analytical studies of TiO₂ NPs and their aggregation, coagulation, flocculation, sedimentation, stabilization under a wide range of conditions and processes. Much attention is paid on sample preparation prior to an analytical procedure, analysis of particle size, morphology, structure, state, chemical composition, surface properties, etc., via measurements of light scattering and zeta potential, microscopy, spectroscopy, and related techniques. Recently, some advanced techniques have also been explored to characterize TiO₂ NPs and their behaviors in the environment. Many issues must be considered including distinction between engineered TiO₂ NPs and their naturally occurring counterparts, lack of reference materials, interlaboratory comparison, when analyzing low concentrations of TiO₂ NPs and their behaviors in complex matrices. No "ideal" technique has emerged as each technique has its own merits, biases, and limitations. Multi-method approach is highlighted to provide in-depth information. Improvements of analytical method for determination of TiO₂ NPs have been recommended to be together with exposure modelers and ecotoxicologists for maximum individual and mutual benefit. Future work should focus on developing analytical technology with the advantages of being reliable, sensitive, selective, reproducible, and capable of in situ detection in complicated sample system.

Secondary Metabolites in the Green Synthesis of Metallic Nanoparticles

The ability of organisms and organic compounds to reduce metal ions and stabilize them into nanoparticles (NPs) forms the basis of green synthesis. To date, synthesis of NPs from various metal ions using a diverse array of plant extracts has been reported. However, a clear understanding of the mechanism of green synthesis of NPs is lacking. Although most studies have neglected to analyze the green-synthesized NPs (GNPs) for the presence of compounds derived from the extract, several studies have demonstrated the conjugation of sugars, secondary metabolites, and proteins in these biogenic NPs. Despite several reports on the bioactivities (antimicrobial, antioxidant, cytotoxic, catalytic, etc.) of GNPs, only a handful of studies have compared these activities with their chemically synthesized counterparts. These comparisons have demonstrated that GNPs possess better bioactivities than NPs synthesized by other methods, which might be attributed to the presence of plant-derived compounds in these NPs. The ability of NPs to bind with organic compounds to form a stable complex has huge potential in the harvesting of precious molecules and for drug discovery, if harnessed meticulously. A thorough understanding of the mechanisms of green synthesis and high-throughput screening of stabilizing/capping agents on the physico-chemical properties of GNPs is warranted to realize the full potential of green nanotechnology.

A Triple Functional Approach To Simultaneously Determine the Type, Concentration, and Size of Titanium Dioxide Particles

The large-scale manufacturing and use of titanium dioxide (TiO₂) particles in food and consumer products significantly increase the likelihood of human exposure and release into the environment. We present a simple and innovative approach to rapidly identify the type (anatase or rutile), as well as to estimate, the size and concentration of TiO₂ particles using Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS). The identification and discrimination of rutile and anatase were based on their intrinsic Raman signatures. The concentration of the TiO₂ particles was determined based on Raman peak intensity. Particle sizes were estimated based on the ratio between the Raman intensity of TiO₂ and the SERS intensity of myricetin bound to the nanoparticles (NPs), which was proven to be independent of TiO₂ nanoparticle concentrations. The ratio that was calculated from the 100 nm particles was used as a cutoff value when estimating the presence of nanosized particles within a mixture. We also demonstrated the practical use of this approach when determining the type, concentration, and size of E171: a mixture that contains TiO₂ particles of various sizes which are commonly used in many food products as food additives. The presence of TiO₂ anatase NPs in E171 was confirmed using the developed approach and was validated by transmission electron micrographs. TiO₂ presence in pond water was also demonstrated to be an analytical capability of this method. Our approach shows great promise for the rapid screening of nanosized rutile and anatase TiO₂ particles in complex matrixes. This approach will strongly improve the measurement of TiO₂ quality during production, as well as the survey capacity and risk assessment of TiO₂ NPs in food, consumer goods, and environmental samples.