2.2.1. EETMSi-Mediated Formation of PBN

The synthesis of PBN was accomplished using [2-(3,4-Epoxycyclohexyl)ethyl]trimethoxysilane(EETMSi) and cyclohexanone from the single precursor potassium ferricyanide via chemical reduction. The homogeneous colloidal sol of Prussian blue nanoparticles (PBN) was prepared by adding 20 µL of EETMSi (0.1 M) to 100 µL of potassium ferricyanide (0.03 M) under stirring conditions. Subsequently, 20 µL of cyclohexanone was added to the reaction mixture; this mixture was kept in an oven at 343 K for 8 h. The blue-colored colloidal suspension of PBN was characterized byX-ray diffraction (XRD), Transmission electron micrpscopy (TEM), etc.

### 2.2.2. EETMSi-Mediated Formation of Prussian Blue Nanoparticles Modified Silica (PBN@SiO2)

Mesoporous silica was used to obtain PBN-confined mesoporous silica (PBN@SiO2). Typical synthesis involved a multistep procedure as follows: At first, 10 mg of mesoporous silica beads were suspended in 100 mL of EETMSi (1.2 M) aqueous solution under constant stirring conditions. After 3 h, un-adsorbed EETMSi was extracted with methanol, followed by centrifugation. 200 mL of potassium ferricyanide [K3Fe(CN)6] aqueous solution (0.03 M) was added to the alkoxysilane-modified SiO<sup>2</sup> suspension under vigorous stirring conditions (800 rpm). Cyclohexanone was added to the alkoxysilane-modified K3Fe(CN)6@SiO<sup>2</sup> suspension under vigorous stirring and left to stand in oven at 338 K overnight. The unreacted K3Fe(CN)<sup>6</sup> and unabsorbed PBN were removed via washing (five times) with methanol/water (2:1) solvent. The residual material was collected after centrifugation; a drying step was subsequently performed.

### 2.2.3. EETMSi-Mediated Formation of PBN@MSNPs

Mesoporous silica nanoparticles (MSNPs) were used to prepare Prussian blue nanoparticleembedded mesoporous silica nanoparticles (PBN@MSNP). Ten milligrams of mesoporous silica nanoparticles (average particle size 200 nm and pore size 6 nm) were suspended in 100 mL of EETMSi (1.2 M) aqueous solution under stirring conditions. After 3 h, un-adsorbed EETMSi was removed with methanol, followed by centrifugation. Two hundred milliliters of potassium ferricyanide aqueous solution (0.03 M) were added to the alkoxysilane-modified MSNP suspension under vigorous stirring conditions (800 rpm). Cyclohexanone was added to the alkoxysilane-modified K3Fe(CN)6@MSNPs suspension under continuous stirring and left to stand in oven at 338 K overnight. The unreacted K3Fe(CN)<sup>6</sup> and unabsorbed PBN were removed via washing (five times) with methanol/water (2:1) solvent. The residual material (PBN@MSNPs) was collected after centrifugation; a drying step was subsequently performed.

### *2.3. Materials Characterization*

The particle size and morphology of as-synthesized PBN/PBN@SiO<sup>2</sup> and PBN@MSNP were analyzed using high-resolution transmission electron microscopy (HRTEM) with 800 and 8100 instruments (Hitachi, Tokyo, Japan) at an acceleration voltage of 200 kV. The topographical properties of as-synthesized PBN over SiO<sup>2</sup> were analyzed using a field emission scanning electron microscopy instrument (FEI (S.E.A.) Pte Ltd., Singapore). The elemental confirmation and mapping analyses were accomplished with an EDX attachment (Oxford Instruments plc, Abingdon, UK). A Rigaku X-ray diffractometer (Rikagu, Tokyo, Japan) with Cu Ka radiation (λ = l.5406 A<sup>0</sup> ) was used to evaluate diffraction data. The XRD analysis was performed over the scan range of 10–90◦ for PBN. FTIR spectra were recorded on an ALFA-ATR Fourier transform infrared spectrometer (Bruker, Ettington, Germany). XPS analysis was performed using an ESCA/AES System (Surface Nano Analysis, GmbH, Berlin, Germany), which was equipped with an Al-Kα (1486.6 eV) X-ray source operating at a power of 385 W and a PHOBIOS 150 3D energy hemispherical analyzer with a delayline detector (SPECS Surface Nano Analysis GmbH, Berlin, Germany). The C-1s peak (284.5 eV)

was used as an internal reference to calibrate the absolute binding energy. The quantitative detection of elements was performed through ICP techniques. Fluorescence analysis was performed using a 7100 spectrophotometer (Hitachi, Tokyo, Japan). Arsenic speciation was performed using high-performance liquid chromatography (HPLC) with a Shim-packed GIST C18 chromatography column encompassing a hydrophobic (non-polar) stationary phase (column length = 75 mm, inner diameter = 7.6 mm) for the determination of all species. Ammonium phosphate solution was used as an eluent for the entire HPLC experiment. The HPLC mobile phases of ammonium phosphate solution with pH 6.9 were prepared by mixing monobasic (NH4H2PO4) and dibasic ((NH4)2HPO4) salt solutions with an appropriate ratio.

### *2.4. Fluorometric Method*

A fluorometric method was used for the determination of As(III) species. Fluorescein (Flo) was used as a probe molecule (λex = 480 nm, λem = 510 nm) for the estimation of As(III) species. The fluorescence experiment was performed under neutral pH (6.8) conditions using Milli-Q water. Different concentrations of As(III) standard solution (10 ppm to 320 ppm) were prepared by adding appropriate amounts of sodium arsenite to Milli-Q water. The result was obtained using the effective concentration of Flo, PBN, and As(III).

### **3. Results and Discussion**

### *3.1. Organotrialkoxysilane-Mediated Synthesis of PBN Analogs*

Organotrialkoxysilane with an amine functional group, APTMS, in the presence of cyclohexanone was previously used for the controlled conversion of a single precursor, K3[Fe(CN)6, into Prussian blue nanoparticles under ambient conditions [20]. Subsequently, 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (EETMSi) was used to make PBN in the absence of cyclohexanone [9]. Although the process enabled efficient conversion of single precursor, K3[Fe(CN)6, into Prussian blue nanoparticles, the duration was substantially longer. Accordingly, we attempted to use cyclohexanone along with EETMSi to obtain PBN from a single precursor pathway. Indeed, the process enabled the rapid formation of PBN, as shown in Figure 1; additional details on this process are provided below.

### 3.1.1. PBN as a Homogeneous Suspension

Slow decomposition under hydrothermal conditions via single precursor synthesis readily produced a blue-colored solution of PBN. The TEM micrographs in Figure 1A,B revealed well-dispersed nanocubes of PBN with an average diameter of 30 nm. The histogram (inset of Figure 1A) shows broad size distribution of crystalline nanoparticles, ranging between 27 and 53 nm. The average width of nanoparticles may be altered by modifying the EETMSi/Fe3+/cyclohexanone feed ratio and thermal conditions. Accordingly, we investigated the role of EETMSi in combination with a ketonic reducing agent. The EDX and TEM data provided information on the chemical composition and nanoparticle structure, respectively. Figure 1D shows the contributions to the EDX spectrum from the Fe Kα peak at 6.4–7.0 keV and 0.9 keV, the Cu peak at 7.8–9.0 keV, and the Si peak at 7.057 keV; Fe peak and the peaks for N, O, and C are also noted. The XRD spectrum shown in Figure 1E reveals nearly all the planes assigned to 2θ values as per JCPDS # 73-0687, 17.4◦ (200), 24.7◦ (220), 35.3◦ (400), 39.6◦ (420), and 43.7◦ (422), 50.0◦ (440), 53.9◦ (600), 57.2◦ (620), 66.1◦ (640), and 68.9◦ (642).
