**1. Introduction**

Large numbers of people in Bangladesh and India are exposed to arsenic contamination in potable water. Metallurgical, agricultural, and industrial processes result in the discharge of arsenic into soil and water [1,2]. Long-term exposure to arsenic, even at low concentrations, can lead to oncological, immunological, neurological, and endocrine effects [3]. The World Health Organization recently set an arsenic limit of 10 µg/L for drinking water (Holm, 2002) [4]. Natural water predominantly contains the inorganic species arsenate [HAsO<sup>4</sup> <sup>2</sup>−, As(V)] and arsenite [AsO<sup>2</sup> −, As(III)]. Inorganic As(III) was noted to be more toxic (10 times), mobile, and water-soluble (4–10 times) than As(V) [5]. The conversion rate of As(III) (arsenite) to As(V) (arsenate) in oxygenated water is a slow process, which depends on certain specific conditions [6]. Consequently, there is an alarming need to develop novel methods for sensing and removal of arsenic from drinking water [7].

Prussian blue nanoparticles (PBN) contain metal in two different oxidation states, Fe+3 and Fe+2; these materials are known for their advanced peroxidase mimetic activity [8–10].

**Citation:** Pandey, P..C.; Shukla, S.; Narayan, R.J. Organotrialkoxysilane-Functionalized Prussian Blue Nanoparticles-Mediated Fluorescence Sensing of Arsenic(III). *Nanomaterials* **2021**, *11*, 1145. https://doi.org/ 10.3390/nano11051145

Academic Editors: Henry Radamson, Guilei Wang and Antonios Kelarakis

Received: 16 February 2021 Accepted: 26 April 2021 Published: 28 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The charge transfer between the two iron species is responsible for the deep blue color of the complex [11]. Bi-metallic coordination compound PBN are a well-known inorganic material for electrocatalytic applications [12–15]. Several reports demonstrated the formation of mixed metal analogues, which involve straightforward replacement of the ferric/ferrous ion with another metal having a similar chemical state [16–18]. The properties of Prussian blue complex can be readily modified depending upon the nature of the constituent metal pair. Iron hexacyanoferrate synthesized via traditional synthetic routes (e.g., coprecipitation and electrosynthesis) do not exhibit appropriate processability for technical applications. We processed PBN from a single precursor involving the active role of the organotrialkoxysilane, which not only controlled the nucleation and solubility but also provided stability to the contents of reaction medium [19]. In addition, the PBN made from a single precursor were found to act as a light quenching material [20]; the photoactivity of the materials was examined using fluorescence imaging. Earlier studies show that organotrialkoxysilanes such as 3-aminopropyltrimethoxysilane (APTMS) allow the conversion of a single precursor, potassium hexacyanoferrate, to Prussian blue; this material was used for electrocatalytic detection of dopamine [21]. We further examined the use of another organotrialkoxysilane, 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (EETMSi), in the presence of cyclohexanone for the controlled synthesis of PBN as a light quenching material.

Several methods, including iron oxide-coated sand, manganese greens, and iron ores, were previously described for arsenic removal [22]. Spectrophotometric and fluorometric methods have been previously studied to estimate the trace amounts of arsenite in water [23–25]. PBN, which include iron of two different oxidation states in a metal framework, may undergo specific interactions with As(III). Thus, we examined the fluorescence quenching ability of the PBN in the presence of arsenic(III). A novel result based on fluorescent sensing of arsenic was recorded, indicating the interaction between PBN and arsenic(III). PBN within a matrix were subsequently studied for use in arsenic removal.

Silica (SiO2) beads are a non-toxic and inexpensive matrix, which may be used as a template to synthesize PBN using organotrialkoxysilane. PBN were inserted into mesoporous SiO2; the PBN became embedded in the accessible SiO<sup>2</sup> pores. The PBN@SiO<sup>2</sup> was used for As(III) removal and its subsequent oxidation into arsenate through an interaction with the iron species in the material. This adsorption–oxidation process was demonstrated with PBN@SiO<sup>2</sup> under different pH conditions to analyze the efficacy of the oxidant system. The high uptake efficiency of PBN@SiO<sup>2</sup> (95%) indicated that this material is attractive for use in As(III) removal. XPS, ICP, and HPLC techniques were used to detect and quantify As(III) species. The PBN@SiO<sup>2</sup> was separated easily through centrifugation; this recycled material also showed As(III) removal activity. The proposed As(III) removal process is more cost effective over those reported to date. The ability to recycle PBN@SiO<sup>2</sup> adds to the economic viability of this process.

### **2. Experimental Section**

### *2.1. Materials*

Potassium ferricyanide was purchased from Merck India (Bengaluru, Karnataka, India). Silica beads (50 µm) and silica nanoparticles (200 nm) were purchased from Sigma-Aldrich (Bengaluru, Karnataka, India). Sodium arsenite was purchased from S D Fine-chem Limited (Mumbai, Maharashtra, India), and Azure-B was obtained from Sisco Research Laboratories Pvt. Ltd. (Mumbai, Maharashtra, India). 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (EETMSi) and cyclohexanone were obtained from Sigma-Aldrich (Bengaluru, Karnataka, India). In addition, the remaining chemicals were of analytical grade and procured from commercial sources. The working solution of As(III) was freshly prepared with Milli-Q water using sodium arsenite (NaAsO2) and stored in a dark freezer. Milli-Q water was used throughout the experiment to avoid interference from contaminants.
