Synthetic Tuning of CoII-Doped Silica Nanoarchitecture Towards Electrochemical Sensing Ability

The present work introduces both synthesis of silica nanoparticles doped with CoII ions by means of differently modified microemulsion water-in-oil (w/o) and Stöber techniques and characterization of the hybrid nanoparticles (CoII@SiO2) by TEM, DLS, XRD, ICP-EOS, SAXS, UV-Vis, and UV-Vis/DR spectroscopy and electrochemical methods. The results reveal the lack of nanocrystalline dopants inside the hybrid nanoparticles, as well as no ligands, when CoII ions are added to the synthetic mixtures as CoII(bpy)3 complexes, thus pointing to coordination of CoII ions with Si-O- groups as main driving force of the doping. The UV-Vis/DR spectra of CoII@SiO2 in the range of d-d transitions indicate that Stöber synthesis in greater extent than the w/o one stabilizes tetrahedral CoII ions versus the octahedral ions. Both cobalt content and homogeneity of the CoII distribution within CoII@SiO2 are greatly influenced by the synthetic technique. The electrochemical behavior of CoII@SiO2 is manifested by one oxidation and two reduction steps, which provide the basis for electrochemical response on glyphosate and HP(O)(OEt)2 with the LOD = 0.1 μM and the linearity within 0.1–80 μM. The Stöber CoII@SiO2 are able to discriminate glyphosate from HP(O)(OEt)2, while the w/o nanoparticles are more efficient but nonselective sensors on the toxicants.


Introduction
Silica nanoparticles provide a good platform for uploading of metal ions and complexes through different synthetic approaches, including doping into silica matrix or deposition at a surface of nanoparticles [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Such hybrid silica nanoparticles uploaded by transition d-metal ions exhibit electrochemical behavior controlled by both inner-sphere environments of the metal ions and nano-architecture of the nanoparticles [14,15,17,18]. Controllable electrochemical behavior is a prerequisite for an application of the hybrid nanoparticles in electrochemical catalysis or sensing. In turn, nanoheterogeneous catalysts and sensors are of particular importance due to their greater stability and reusability versus the molecular complexes [14,15,17,18]. The substrate dependent electrochemical behavior of transition metal ions and complexes is the basis for electrochemical detection of many pollutants, including organophosphorous ones [19][20][21][22][23][24][25][26][27][28][29]. Electrochemical methods have many advantages over other sensing techniques such as reproducibility, good stability, high sensitivity, Et 4 NBF 4 was obtained by mixing an aqueous solution of Et 4 NOH (30-35%) with HBF 4 to a neutral indicator reaction. Et 4 NBF 4 precipitated from the reaction mixture as white crystals, which were separated by filtering. The powder salt was further recrystallized from diethyl ether and dried for 2 to 3 days in a vacuum at 55 • C for dehydration.

Silica Nanoparticles Preparation
Water-in-oil (w/o) microemulsion procedure [44] was the following. A mixture of Triton X-100 (2.38 g), n-heptanol (2.29 mL), cyclohexane (9.32 mL), TEOS (0.2 mL), and 1.1 mL of bidistilled water (for "empty" silica nanoparticles) or aqueous solution (1.1 mL) of the Co II -precursor (Co(bpy) 3 (BF 4 ) 2 or CoCl 2 , C = 4.5×10 −3 mol·L −1 ) was prepared and stirred for 30 min. The obtained w/o microemulsion was mixed with a microemulsion containing Triton X-100 (2.38 g), n-heptanol (2.29 mL), cyclohexane (9.32 mL) and aqueous solutions of NH 3 (28-30%) with stirring. After 24 h of stirring, both Co II -doped and empty silica nanoparticles were precipitated from the microemulsion by adding acetone with further centrifugation. The nanoparticle precipitate was washed by solutions of ethanol-acetone (1:1), ethanol (two times), and water (several times) to remove any residual surfactant molecules and organic solvents. Physically adsorbed Co II ions or complexes and Triton X-100 molecules at nanoparticles surface were removed by ultrasonication during the washing procedure. Pink-colored Co II -doped and white "empty" silica nanoparticles were obtained.
Stöber synthesis [14] was done through the following steps. A solution of TEOS (1.14 mL) in EtOH (11.36 mL) was added to NH 4 OH (28-30%) (0.38 mL) in EtOH (8.05 mL)-H 2 O (4.05 mL) at the speed 2 mL per min (thought syringe pump) under continuous stirring (750 rpm). After 6 min, the solution of [Co(bpy) 3 ](BF 4 ) 2 or CoCl 2 in EtOH (2.5 mL, C = 4.5×10 −3 mol·L −1 ) was injected into this mixture. In the case of the "empty" silica nanoparticles, the solution of TEOS (1.14 mL) in EtOH (11.36 mL) was added to NH 4 OH (28-30%) (0.38 mL) in EtOH (10.55 mL)-H 2 O (4.05 mL) at the speed 2 mL per min (thought syringe pump) under continuous stirring (750 rpm). After 6 h of stirring, silica nanoparticles were precipitated by centrifuging and washing several times by ethanol and water. To remove physically absorbed Co II complexes or ions from the particles surfaces, an ultrasonication was used during the washing procedure. Blue-colored Co II -doped and white "empty" silica nanoparticles were obtained.
The electrochemical, PXRD, SAXS, IR, and Raman experiments were performed with dried samples of the silica nanoparticles.

Samples Characterization
Transmission electron microscopy (TEM) measurements were carried out on Hitachi HT7700 transmission electron microscope (Chiyoda-ku, Japan) with energy-dispersive X-ray detector from Thermo Scientific. The accelerating voltage was equal to 80 kV. The accumulation time of one spectrum was 300 s.
Co and Si were identified in the silica nanoparticles colloids using a simultaneous inductively coupled plasma atomic emission spectrometry (ICP-AES) model iCAP 6300 DUO by Varian Thermo Scientific Company equipped with a CID detector. This spectrometer enables the simultaneous Nanomaterials 2020, 10, 1338 4 of 19 measurement of peak heights within the 166 to 867 nm range. The optical resolution is less than 0.007 nm to 200 nm. As for the working frequency, it is 27.12 MHz. Together, the radial and axial view configurations enable optimal peak height measurements with suppressed spectral noises.
UV-Vis spectra of solutions and silica nanoparticles dispersions were recorded on Specord R 50 Plus (Analytikjena, Germany). SNs dispersions were ultrasonicated within 10 min before using.
Powdered samples were characterized by UV-Vis/DR technique using a Jasco V-650 spectrophotometer (Jasco International Co. Ltd., Hachioji, Tokyo, Japan) equipped with an integrating sphere accessory for diffuse reflectance spectra acquisition. BaSO 4 powder was used as the reference for baseline correction.
The dynamic light scattering (DLS) measurements were performed by means of the Malvern Mastersize 2000 particle analyzer (Malvern, UK). A He-Ne laser operating at 633 nm wavelength and emitting vertically polarized light was used as a light source. The measured autocorrelation functions were analyzed by Malvern DTS software and the second-order cumulant expansion methods. The effective hydrodynamic radius (RH) was calculated by the Einstein-Stokes relation from the first cumulant: D = kBT/6phRH, where D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, and h is the viscosity. The diffusion coefficient was measured at least three times for each sample. The average error in these experiments was approximately 4%. All samples were prepared from the bidistilled water with prior filtering through the PVDF membrane using the Syringe Filter (0.45 µm). Zeta potential "Nano-ZS" (MALVERN) using laser Doppler velocimetry and phase analysis light scattering was used for zeta potential measurements. Silica nanoparticles dispersions were ultrasonicated within 10 min before using.
IR spectra of solid samples have been registered using a Bruker Vector-27 FTIR spectrometer in the 400-4000 cm −1 range (optical resolution 4 cm −1 ). The samples were prepared as KBr pellets.
Raman spectra were registered at room temperature using a BRUKER RAM II module attached to a BRUKER VERTEX 70 FTIR spectrometer (excitation 1064 nm, Ge detector at liquid nitrogen temperature, back-scattering configuration; range 10-4000 cm −1 , optical resolution 4 cm −1 , scan number 1024).
Electrochemical measurements were taken on a BASiEpsilonE2P electrochemical analyzer (West Lafayette, IN, USA). The program concerned Epsilon-ECUSB-V200 waves. A conventional three-electrode system was used with glassy carbon for carbon paste electrode (CPE) solutions for powder samples as the working electrode, the An Fc/Fc system serving as reference electrode, and a Pt wire as the counter electrode. In addition, 0.1 M [Et 4 N][BF 4 ] was used as the supporting electrolyte to determine the current voltage characteristics. The solvent for all measurements was water. To study the powder samples, a modified CPE working electrode was used, which was prepared as follows: the carbon particles/phosphonium salt (dodecyl(tri-tert-butyl)phosphonium tetrafluoroborate) composite electrode was prepared using a grinding a mixture of graphite powder and phosphonium salt with a 90/10 (w/w) ratio in mortar giving it a homogeneous mass [45]. A modified electrode was also devised in a similar manner except that a portion (ca. 5%) of the graphite powder was replaced by the CPs under study. As a result, a portion of the resulting paste was packed firmly into the (3 mm in diameter) Teflon holder cavity.
Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Advance diffractometer equipped with Vario attachment and Vantec linear PSD, using Cu radiation (40 kV, 40 mA) monochromated by the curved Johansson monochromator (λ Cu K α1 1.5406 Å). Room-temperature data were collected in the reflection mode with a flat-plate sample. The samples were loaded on a standard zero diffraction silicon plate, which was kept spinning (15 rpm) throughout the data collection. Patterns were recorded in the 2Θ range between 3 • and 93 • , in 0.008 • steps, with a step time of 0.1-4.0 s. Several diffraction patterns in various experimental modes were collected and summed for the sample. Processing of the obtained data performed using EVA software packages [46].
Small angle X-Ray scattering (SAXS) data for samples were collected with the Bruker AXS Nanostar system using CuKα (λ 1.5418Å) radiation from a 2.2 kW X-ray tube (40 kV, 35 mA) coupled with Gobbel mirrors optics and a HiStar 2D area detector. The beam was collimated using three pinholes Nanomaterials 2020, 10, 1338 5 of 19 with apertures of 800, 450, and 700 µm. The instrument was operated with a sample-to-detector distance of 63.5 cm to provide data at angles 0.1 • < 2θ< 4.8 • , which correspond to 0.007 Å−1 < s < 0.34 Å−1. The value of s is proportional to the inverse of the length scale (s = (4π/λ)sin(θ) in units of Å −1 ). Scattering patterns were obtained for the samples at 23 • C in an evacuated chamber. The measurements were performed in transmission mode with the use of glass capillaries filled by powder samples. The capillaries (2 mm diameter) were sealed and put into evacuated chamber by means of the holders. For each sample, several experiments were performed, allowing for controlling the quality of the experiments. The results of the experiments are summarized, so that the total time of each experiment was equal to 30,000 s. The 2D scattering patterns were integrated using the SAXS program package [47]. Calculation of structural parameters, simulation, and graphical representation of the results were performed using a PRIMUS [48] program package.

Size, Cobalt Content, and Spectral Properties of the Hybrid Nanoparticles (CoII@SiO2) in Correlation with the Synthetic Conditions
Stöber and water-in-oil microemulsion (w/o) techniques provide a facile approach for encapsulation of charged metal complexes into silica nanoparticles [2,9,[14][15][16][38][39][40]. The use of the both techniques results in the synthesis of the hybrid silica nanoparticles. Figure 1 illustrates the TEM images of the hybrid nanoparticles, while their diameters evaluated from the images and the Si:Co ratios calculated from the ICP-EOS data are collected in Table 2. The data in Table 1 indicate that the size and the content of Co II evaluated by the Si:Co ratios are to a major extent dependent on the synthetic technique, while smaller differences derive from the doping of Co II (bpy) 3 or CoCl 2 . The difference in the synthetic methods is illustrated by Scheme 1. The size of the nanoparticles synthesized by the w/o technique is controlled by the TX-100-based reverse micelles (Scheme 1). Thus, water:oil volume ratios along with surfactant and co-surfactant concentrations have a great impact on the size control in the framework of the w/o technique. The sizes of silica nanoparticles grown by the Stöber method are mainly affected by concentrations of TEOS and ammonia, while the ethanol:water volume ratio plays a smaller role in controlling the size (Scheme 1). Moreover, the water soluble Co II -dopants are concentrated within nano-droplets of the reverse micelles, which explains the greater uploading by Co II ions for the nanoparticles synthesized by the w/o procedure in comparison with those obtained by the Stöber method ( Table 2, Scheme 1). It is also worth noting that the color of both aqueous colloids and dried samples of the hybrid nanoparticles are pink when they are synthesized by the w/o, while the Stöber procedure results in the blue nanoparticles. Similar to the above-mentioned factors the color of the hybrid nanoparticles seems to be independent from their doping with Co II (bpy) 3 or CoCl 2 . Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 20 pinholes with apertures of 800, 450, and 700 µm. The instrument was operated with a sample-todetector distance of 63.5 cm to provide data at angles 0.1°< 2θ< 4.8°, which correspond to 0.007 Å−1 < s < 0.34 Å−1. The value of s is proportional to the inverse of the length scale (s = (4π/λ)sin(θ) in units of Å −1 ). Scattering patterns were obtained for the samples at 23 °C in an evacuated chamber. The measurements were performed in transmission mode with the use of glass capillaries filled by powder samples. The capillaries (2 mm diameter) were sealed and put into evacuated chamber by means of the holders. For each sample, several experiments were performed, allowing for controlling the quality of the experiments. The results of the experiments are summarized, so that the total time of each experiment was equal to 30,000 s. The 2D scattering patterns were integrated using the SAXS program package [47]. Calculation of structural parameters, simulation, and graphical representation of the results were performed using a PRIMUS [48] program package.

Size, Cobalt Content, and Spectral Properties of the Hybrid Nanoparticles (CoII@SiO2) in Correlation with the Synthetic Conditions
Stöber and water-in-oil microemulsion (w/o) techniques provide a facile approach for encapsulation of charged metal complexes into silica nanoparticles [2,9,[14][15][16][38][39][40]. The use of the both techniques results in the synthesis of the hybrid silica nanoparticles. Figure 1 illustrates the TEM images of the hybrid nanoparticles, while their diameters evaluated from the images and the Si:Co ratios calculated from the ICP-EOS data are collected in Table 2. The data in Table 1 indicate that the size and the content of Co II evaluated by the Si:Co ratios are to a major extent dependent on the synthetic technique, while smaller differences derive from the doping of Co II (bpy)3 or CoCl2. The difference in the synthetic methods is illustrated by Scheme I. The size of the nanoparticles synthesized by the w/o technique is controlled by the TX-100-based reverse micelles (Scheme I). Thus, water:oil volume ratios along with surfactant and co-surfactant concentrations have a great impact on the size control in the framework of the w/o technique. The sizes of silica nanoparticles grown by the Stöber method are mainly affected by concentrations of TEOS and ammonia, while the ethanol:water volume ratio plays a smaller role in controlling the size (Scheme 1). Moreover, the water soluble Co II -dopants are concentrated within nano-droplets of the reverse micelles, which explains the greater uploading by Co II ions for the nanoparticles synthesized by the w/o procedure in comparison with those obtained by the Stöber method (Table 2, Scheme 1). It is also worth noting that the color of both aqueous colloids and dried samples of the hybrid nanoparticles are pink when they are synthesized by the w/o, while the Stöber procedure results in the blue nanoparticles. Similar to the above-mentioned factors the color of the hybrid nanoparticles seems to be independent from their doping with Co II (bpy)3 or CoCl2.   The presence of bpy ligands both coordinated with Co(II) and as "free" molecules inside silica nanoparticles can be revealed by UV-Vis spectroscopy measurements in both supernatants ( Figure  2b) and aqueous dispersions of the synthesized silica nanoparticles (Figure 2a). The analysis of the UV-Vis spectral data reveals the lack of the bands peculiar for Co II (bpy)3 in the aqueous colloids for both types of Co II @SiO2 (curves 2 and 3 in Figure 2a). The supernatant after the synthesis by the Stöber procedure reveals the release of bpy from the complex (Figure 2b), while the electronic absorption bands of Triton X-100 mask the presence of bpy in the supernatant after the synthesis by the w/o method ( Figure S1). Moreover, its concentration in the supernatant indicates the degradation of about 57% of Co II (bpy)3 added to the synthetic mixture. The presence of bpy ligands both coordinated with Co(II) and as "free" molecules inside silica nanoparticles can be revealed by UV-Vis spectroscopy measurements in both supernatants ( Figure 2b) and aqueous dispersions of the synthesized silica nanoparticles (Figure 2a). The analysis of the UV-Vis spectral data reveals the lack of the bands peculiar for Co II (bpy) 3 in the aqueous colloids for both types of Co II @SiO 2 (curves 2 and 3 in Figure 2a). The supernatant after the synthesis by the Stöber procedure reveals the release of bpy from the complex (Figure 2b), while the electronic absorption bands of Triton X-100 mask the presence of bpy in the supernatant after the synthesis by the w/o method ( Figure S1). Moreover, its concentration in the supernatant indicates the degradation of about 57% of Co II (bpy) 3 added to the synthetic mixture.
Thus, the color changes during the synthesis of the nanoparticles by the w/o and Stöber procedures derive from the transformation of the spectral pattern from Co II (bpy) 3 to that of bpy ( Figure 3). In particular, the yellow color seems to remain unchanged in aqueous or alcohol solution of Co(bpy) 3 (BF 4 ) 2 exactly after addition of tetraethoxysilane and ammonia (4-8 in Figure 3), which is confirmed by the small changes in the spectral pattern of Co II (bpy) 3 . The color changes followed by the spectral transformations from that of Co II (bpy) 3 to that of bpy (3 and 1 in Figure 3) is observed within 6 or 30 min after beginning of the Stöber (9 in Figure 3) or w/o synthesis. The time durations for 6 and 30 min correlate with the appearance of silica seeds in the framework of Stöber [14,39] and w/o [49] procedures. It is worth assuming that the growth of silica seeds with Si-Ogroups exposed at their surface provides a reason for the transformations of Co II (bpy) 3 to bpy and Co II ions due to a complex formation of the latter with Si-Ogroups of the silica seeds. The assumption can be confirmed by comparison of the UV-Vis diffuse reflectance spectra in the long-wave spectral range derived from the d-d transitions of the hybrid nanoparticles doped with Co(dipy) 3 (BF 4 ) 2 or CoCl 2 ( Figure 4). For these purposes, the UV-Vis/DR spectra of Co II @SiO 2 were recorded for their dried samples along with the powder of Co(dipy) 3 (BF 4 ) 2 ( Figure 4) and "empty" silica spheres (SiO 2 ) ( Figure S2) for the comparison. The latter were synthesized by the same w/o and Stöber procedures without the addition of the Co II -dopants. The characterization of SiO 2 synthesized in the same synthetic conditions as the Co II @SiO 2 by TEM indicate that their sizes are greatly affected by the synthetic method (48 ± 5 and 97 ± 5 nm for SiO 2 synthesized by w/o and Stöber methods correspondingly), while the effect of the dopants is rather small (compare with the sizes in Table 2). Scheme 1. Schematic representation of different techniques for synthesis of Co II @SiO2.
The presence of bpy ligands both coordinated with Co(II) and as "free" molecules inside silica nanoparticles can be revealed by UV-Vis spectroscopy measurements in both supernatants ( Figure  2b) and aqueous dispersions of the synthesized silica nanoparticles (Figure 2a). The analysis of the UV-Vis spectral data reveals the lack of the bands peculiar for Co II (bpy)3 in the aqueous colloids for both types of Co II @SiO2 (curves 2 and 3 in Figure 2a). The supernatant after the synthesis by the Stöber procedure reveals the release of bpy from the complex (Figure 2b), while the electronic absorption bands of Triton X-100 mask the presence of bpy in the supernatant after the synthesis by the w/o method ( Figure S1). Moreover, its concentration in the supernatant indicates the degradation of about 57% of Co II (bpy)3 added to the synthetic mixture. Thus, the color changes during the synthesis of the nanoparticles by the w/o and Stöber procedures derive from the transformation of the spectral pattern from Co II (bpy)3 to that of bpy ( Figure 3). In particular, the yellow color seems to remain unchanged in aqueous or alcohol solution of Co(bpy)3(BF4)2 exactly after addition of tetraethoxysilane and ammonia (4-8 in Figure 3), which is confirmed by the small changes in the spectral pattern of Co II (bpy)3. The color changes followed by the spectral transformations from that of Co II (bpy)3 to that of bpy (3 and 1 in Figure 3) is observed within 6 or 30 min after beginning of the Stöber (9 in Figure 3) or w/o synthesis. The time durations for 6 and 30 min correlate with the appearance of silica seeds in the framework of Stöber [14,39] and w/o [49] procedures. It is worth assuming that the growth of silica seeds with Si-Ogroups exposed at their surface provides a reason for the transformations of Co II (bpy)3 to bpy and Co II ions due to a complex formation of the latter with Si-Ogroups of the silica seeds. The assumption can be confirmed by comparison of the UV-Vis diffuse reflectance spectra in the long-wave spectral range derived from the d-d transitions of the hybrid nanoparticles doped with Co(dipy)3(BF4)2 or CoCl2 ( Figure 4). For these purposes, the UV-Vis/DR spectra of Co II @SiO2 were recorded for their dried samples along with the powder of Co(dipy)3(BF4)2 ( Figure 4) and "empty" silica spheres (SiO2) ( Figure S2) for the comparison. The latter were synthesized by the same w/o and Stöber procedures without the addition of the Co II -dopants. The characterization of SiO2 synthesized in the same synthetic conditions as the Co II @SiO2 by TEM indicate that their sizes are greatly affected by the synthetic method (48 ± 5 and 97 ± 5 nm for SiO2 synthesized by w/o and Stöber methods correspondingly), while the effect of the dopants is rather small (compare with the sizes in Table 2). The spectra in Figure 4 indicate the difference in the spectral pattern within 500-700 nm for the Co II @SiO2 synthesized by the w/o and Stöber procedures, while the nature of the Co II -precursor (Co(bpy)3(BF4)2 or CoCl2) insignificantly affects the d-d transitions of the Co II @SiO2. The spectra of Co (II) @SiO2 synthesized with the use of Co II (bpy)3 as precursor (2 and 3 in Figure 4) differ from the one of Co II (bpy)3 complex in the range associated with d-d transitions, but demonstrate the band at contribution of the octahedral Co II chromophores in the w/o Co II @SiO2 versus the Stöber Co II @SiO2. It is noteworthy that the coordinative bonds between the Co II ions and Si-Ogroups at the silica surface are the key reason for the formation of the Co II @SiO2 by the w/o and the Stöber procedures. However, it is worth assuming that the extents of tetrahedral and octahedral Co 2+ chromophores are greatly affected by the aqueous or alcohol environment of Co II ions in the frameworks of the w/o or Stöber techniques (Scheme 1). The infrared and Raman spectra of Co II @SiO2 nanoparticles ( Figure 5) synthesized by both methods are very similar and demonstrate characteristic for silica nanoparticles bands: 475 cm −1 , attributed to bending modes between the Si-O-Si bonds, 800 cm −1 , assigned to the symmetric stretching vibration between Si-O-Si bonds, ~960 cm −1 that is often linked with the stretching vibration of Si-OH bond and ~1100 cm −1 , which corresponds to the asymmetric stretching vibration of Si-O-Si bond in the SiO4 tetrahedron as reported in literature [41,[57][58][59][60]. The IR spectra of NPs obtained by the w/o technique (cyan and red in Figure 5a) contain additional bands at ~1370, 1701, and 2930 cm −1 most probably associated with CH bending and C=O and alkyl CH stretching vibrations of acetone and TX-100 molecules (cyan and red in Figure 5a). The presence of TX-100 molecules derives from their efficient adsorption at the surface of silica nanoparticles (Scheme I) leading to the so-called surfactant-based corona similar with the previously reported F-127-based . UV-Vis diffuse reflectance spectra of: Co II (bpy) 3 (1); and Co II @SiO 2 synthesized by different techniques: Stöber with Co II (bpy) 3 (2) and CoCl 2 (4); w/o with Co II (bpy) 3 (3) and CoCl 2 (5).
The spectra in Figure 4 indicate the difference in the spectral pattern within 500-700 nm for the Co II @SiO 2 synthesized by the w/o and Stöber procedures, while the nature of the Co II -precursor (Co(bpy) 3 (BF 4 ) 2 or CoCl 2 ) insignificantly affects the d-d transitions of the Co II @SiO 2 . The spectra of Co (II) @SiO 2 synthesized with the use of Co II (bpy) 3 as precursor (2 and 3 in Figure 4) differ from the one of Co II (bpy) 3 complex in the range associated with d-d transitions, but demonstrate the band at 300 nm, absent in the spectra of Co (II) @SiO 2 with CoCl 2 (curve 4 and 5 in Figure 4), associated most probably with "free" bpy. The spectra of the Stöber Co II @SiO 2 (curve 2 and 4 in Figure 4) demonstrate three strong bands at 528, 580, and 638 nm, which are characteristic for 4 A 2 (F) → 4 T 1 (P) transitions in tetrahedral Co II chromophores [50][51][52][53]. It should be mentioned that the same spectral pattern (bands at 547, 585, 633 nm) has been observed for tetrahedral Co II ion coordinated to four oxygen atoms in 1D coordination polymer of Co(fcdHp), based on 1,1 -ferrocenylenbis(H-phosphinic) acid (H2fcdHp) [54]. The similar bands in this region (curve 3 and 5 in Figure 4) are not so pronounced in the spectra of the w/o Co II @SiO 2 . They also demonstrate another rearrangement of intensities. Most probably, the spectral pattern of the w/o Co II @SiO 2 samples derives from Co II ions in octahedral environment, for which weak bands at ca. 500-580 nm assigned to 4 T 1g → 4 A 2g (F) transitions are characteristic [50,53,55]. The absorbance intensity of tetrahedral Co II is more intense (∼100 times) than that of octahedral Co II due to the lack of an inversion center in the former symmetry [56]. Therefore, the contribution from the octahedral Co II to the spectral pattern could be masked by the bands arisen from the tetrahedral Co II chromophores. Nevertheless, both rearrangement of bands intensities at 500-700 nm and their lowering in spectra 3 and 5 compared to 2 and 4 ( Figure 4) allows one to suggest the greater contribution of the octahedral Co II chromophores in the w/o Co II @SiO 2 versus the Stöber Co II @SiO 2 . It is noteworthy that the coordinative bonds between the Co II ions and Si-Ogroups at the silica surface are the key reason for the formation of the Co II @SiO 2 by the w/o and the Stöber procedures. However, it is worth assuming that the extents of tetrahedral and octahedral Co 2+ chromophores are greatly affected by the aqueous or alcohol environment of Co II ions in the frameworks of the w/o or Stöber techniques (Scheme 1).
The infrared and Raman spectra of Co II @SiO 2 nanoparticles ( Figure 5) synthesized by both methods are very similar and demonstrate characteristic for silica nanoparticles bands: 475 cm −1 , attributed to bending modes between the Si-O-Si bonds, 800 cm −1 , assigned to the symmetric stretching vibration between Si-O-Si bonds,~960 cm −1 that is often linked with the stretching vibration of Si-OH bond and~1100 cm −1 , which corresponds to the asymmetric stretching vibration of Si-O-Si bond in the SiO 4 tetrahedron as reported in literature [41,[57][58][59][60]. The IR spectra of NPs obtained by the w/o technique (cyan and red in Figure 5a) contain additional bands at~1370, 1701, and 2930 cm −1 most probably associated with CH bending and C=O and alkyl CH stretching vibrations of acetone and TX-100 molecules (cyan and red in Figure 5a). The presence of TX-100 molecules derives from their efficient adsorption at the surface of silica nanoparticles (Scheme 1) leading to the so-called surfactant-based corona similar with the previously reported F-127-based corona [61] of the silica nanoparticles synthesized by the w/o procedure with F-127 as the surfactant. Raman spectra of Stöber Co II @SiO 2 based on Co II (bpy) 3 contain very weak bands of bpy at~1600 and 1570 cm −1 (magenta in Figure 5b), which are absent in the spectrum of "empty" SiO 2 (black in Figure 5b), and are not visible in their IR spectra (magenta in Figure 5a). Raman spectra of Stöber Co II @SiO2 based on Co II (bpy)3 contain very weak bands of bpy at ~1600 and 1570 cm −1 (magenta in Figure 5b), which are absent in the spectrum of "empty" SiO2 (black in Figure  5b), and are not visible in their IR spectra (magenta in Figure 5a).

Nanoarchitecture of Co II @SiO2 and SiO2 in Correlation with the Synthetic Procedure
Electrochemical behavior of metal ions encapsulated into silica nanoparticles is affected by the inner-sphere environment of metal ions and their preferable localization within interior (core) or exterior (shell) zones of the hybrid nanoparticles [15,62]. Thus, nanoarchitecture of the hybrid nanoparticles manifested by different distribution of the dopant within their core and shell zones was studied by powder X-ray diffraction (PXRD) and small angle X-ray scattering (SAXS) techniques. Combination of the techniques was used to reveal both supramolecular and nano-structuring in the dried samples of Co II @SiO2 and SiO2 synthesized by the both methods. It is well-known that silica spheres synthesized by w/o or Stöber techniques are manifested by widened peaks in PXRD spectra, while the presence of nano-crystallites derived from the cobalt salts or oxides can be revealed by the appearance of the specific peaks [41,62,63]. Literature data indicate that the amorphous nature of silica remains practically unchanged under coordination of Co(II) ions with Si-Ogroups, while the narrow peaks revealing the nanocrystallites of cobalt oxide derive from the extra-amounts of the doped cobalt salt [41,62]. Such nanocrystallites can be revealed by comparative analysis of PXRD data of Co II @SiO2 nanoparticles synthesized by both methods and their empty analogous (SiO2). The PXRD spectra recorded for the dried samples of Co II @SiO2 and SiO2 nanoparticles synthesized by the both methods ( Figure 6) demonstrate the PXRD patterns peculiar for amorphous silica nanoparticles, which agrees well with the concentration conditions in the synthesis of the Co II @SiO2 (see the Exp. Section for more details). However, the weak widened peaks in the range of small angles revealed in the PXRD curves of Stöber Co II @SiO2 and SiO2 derive from the inclusions of paracrystalline phase with short-and medium-range ordering into the amorphous nanoparticles. . IR (a) and Raman (b) spectra of Co II (bpy) 3 (orange), SiO 2 (black) and Co II @SiO 2 synthesized by the w/o on the basis of Co II (bpy) 3 (cyan) and CoCl 2 (red) and by Stöber on the basis of Co II (bpy) 3 (magenta) and CoCl 2 (blue).

Nanoarchitecture of Co II @SiO 2 and SiO 2 in Correlation with the Synthetic Procedure
Electrochemical behavior of metal ions encapsulated into silica nanoparticles is affected by the inner-sphere environment of metal ions and their preferable localization within interior (core) or exterior (shell) zones of the hybrid nanoparticles [15,62]. Thus, nanoarchitecture of the hybrid nanoparticles manifested by different distribution of the dopant within their core and shell zones was studied by powder X-ray diffraction (PXRD) and small angle X-ray scattering (SAXS) techniques. Combination of the techniques was used to reveal both supramolecular and nano-structuring in the dried samples of Co II @SiO 2 and SiO 2 synthesized by the both methods. It is well-known that silica spheres synthesized by w/o or Stöber techniques are manifested by widened peaks in PXRD spectra, while the presence of nano-crystallites derived from the cobalt salts or oxides can be revealed by the appearance of the specific peaks [41,62,63]. Literature data indicate that the amorphous nature of silica remains practically unchanged under coordination of Co(II) ions with Si-Ogroups, while the narrow peaks revealing the nanocrystallites of cobalt oxide derive from the extra-amounts of the doped cobalt salt [41,62]. Such nanocrystallites can be revealed by comparative analysis of PXRD data of Co II @SiO 2 nanoparticles synthesized by both methods and their empty analogous (SiO 2 ). The PXRD spectra recorded for the dried samples of Co II @SiO 2 and SiO 2 nanoparticles synthesized by the both methods ( Figure 6) demonstrate the PXRD patterns peculiar for amorphous silica nanoparticles, which agrees well with the concentration conditions in the synthesis of the Co II @SiO 2 (see the Exp. Section for more details). However, the weak widened peaks in the range of small angles revealed in the PXRD curves of Stöber Co II @SiO 2 and SiO 2 derive from the inclusions of paracrystalline phase with short-and medium-range ordering into the amorphous nanoparticles. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 20 Figure 6. PXRD patterns: black-w/o SiO2, orange-Stöber SiO2, red-w/o Co II @SiO2, green-Stöber Co II @SiO2. The curves are shifted relative to each other along the intensity axis for clarity.
SAXS has been previously documented as successful technique to reveal nano-structuring deriving from silica nanoparticles, as well as inhomogeneity in electron density distribution within nanoparticles [41,[64][65][66]. It is worth noting in this connection that the sizes of Co II @SiO2 and SiO2 nanoparticles synthesized by Stöber method (Tables 2 and 3) lie above the values available for correct evaluation by the method (1-60 nm), while the sizes of smaller Co II @SiO2 and SiO2 nanoparticles synthesized by the w/o procedure can be evaluated from the SAXS measurements. However, the Xray scattering profiles of the Co II @SiO2 and SiO2 ( Figure S3) reveal their nanostructuring manifested by the presence of nano-aggregates with the sizes below 60 nm even for the Stöber Co II @SiO2 and SiO2. The nanostructuring arisen from inhomogeneity of electron density distribution within the silica nanoparticles may be quantitatively described in a "core-shell" framework. However, the shell zones of such "core-shell" nanoparticles along with the interparticle space can be considered as isotropic homogeneous medium with the electron density lower than that of the core zone. It is worth assuming that the core zones of the Co II @SiO2 and SiO2 as nanoparticles encapsulated into an isotropic medium with distances between their centers above the sizes of the core zones. Thus, the quantitative analysis of the SAXS data was performed in the assumption that the scattering results from noninteracting spherical nanoparticles uniformly enclosed in a homogeneous medium which provides their local monodisperse environment. Two-dimensional scattering curves were obtained by averaging of the eight X-ray scattering measurements for both hybrid and empty silica nanoparticles, while no effect of the prolonged X-ray irradiation on the scattering data was revealed ( Figure S4). Table 3 collects the calculated from the SAXS data parameters characterizing nanoparticles, such as radius of gyration (Rg and Rg*) evaluated by the Guinier method and from analysis of distance distribution function respectively, biggest distances in the particles (Dmax), and average diameter of the particles in a sphere-shaped model framework (ds, ds = √(5/3) × Rg × 2). The latter can be compared with the d-values evaluated by TEM, which are also collected in Table 2 to facilitate the comparison. This difference is too great to be explained by the difference in the techniques. The ds values calculated from SAXS are at the level of 33-38 nm for Co II @SiO2 and SiO2 nanoparticles synthesized by the w/o technique; these values are lower than the d values evaluated by TEM on about 10 nm (Table 3). In the framework of the aforesaid assumption, the sizes revealed by the SAXS measurements refer to the core zones of the w/o Co II @SiO2 and SiO2. This doesn't exclude the presence of an exterior layer exhibiting smaller electronic density than the silica spheres. The above-mentioned TX-100-corona at the surface of w/o Co II @SiO2 and SiO2 is one of the reasons for an exterior layer with the electron density smaller than that arisen from the silica matrix. Therefore, the deviation between the ds and d values can be explained by the fact that the sizes revealed by the TEM images derive from both core and shell zones of the w/o Co II @SiO2 and SiO2. It is also worth noting that the Dmax SAXS has been previously documented as successful technique to reveal nano-structuring deriving from silica nanoparticles, as well as inhomogeneity in electron density distribution within nanoparticles [41,[64][65][66]. It is worth noting in this connection that the sizes of Co II @SiO 2 and SiO 2 nanoparticles synthesized by Stöber method (Tables 2 and 3) lie above the values available for correct evaluation by the method (1-60 nm), while the sizes of smaller Co II @SiO 2 and SiO 2 nanoparticles synthesized by the w/o procedure can be evaluated from the SAXS measurements. However, the X-ray scattering profiles of the Co II @SiO 2 and SiO 2 ( Figure S3) reveal their nanostructuring manifested by the presence of nano-aggregates with the sizes below 60 nm even for the Stöber Co II @SiO 2 and SiO 2 . The nanostructuring arisen from inhomogeneity of electron density distribution within the silica nanoparticles may be quantitatively described in a "core-shell" framework. However, the shell zones of such "core-shell" nanoparticles along with the interparticle space can be considered as isotropic homogeneous medium with the electron density lower than that of the core zone. It is worth assuming that the core zones of the Co II @SiO 2 and SiO 2 as nanoparticles encapsulated into an isotropic medium with distances between their centers above the sizes of the core zones. Thus, the quantitative analysis of the SAXS data was performed in the assumption that the scattering results from noninteracting spherical nanoparticles uniformly enclosed in a homogeneous medium which provides their local monodisperse environment. Two-dimensional scattering curves were obtained by averaging of the eight X-ray scattering measurements for both hybrid and empty silica nanoparticles, while no effect of the prolonged X-ray irradiation on the scattering data was revealed ( Figure S4). Table 3 collects the calculated from the SAXS data parameters characterizing nanoparticles, such as radius of gyration (R g and R g *) evaluated by the Guinier method and from analysis of distance distribution function respectively, biggest distances in the particles (D max ), and average diameter of the particles in a sphere-shaped model framework (d s , d s = . The latter can be compared with the d-values evaluated by TEM, which are also collected in Table 2 to facilitate the comparison. This difference is too great to be explained by the difference in the techniques. The d s values calculated from SAXS are at the level of 33-38 nm for Co II @SiO 2 and SiO 2 nanoparticles synthesized by the w/o technique; these values are lower than the d values evaluated by TEM on about 10 nm (Table 3). In the framework of the aforesaid assumption, the sizes revealed by the SAXS measurements refer to the core zones of the w/o Co II @SiO 2 and SiO 2 . This doesn't exclude the presence of an exterior layer exhibiting smaller electronic density than the silica spheres. The above-mentioned TX-100-corona at the surface of w/o Co II @SiO 2 and SiO 2 is one of the reasons for an exterior layer with the electron density smaller than that arisen from the silica matrix. Therefore, the deviation between the d s and d values can be explained by the fact that the sizes revealed by the TEM images derive from both core and shell zones of the w/o Co II @SiO 2 and SiO 2 . It is also worth noting that the D max values exceed the d s ones on no more than 5 nm (Table 3), which, in turn, indicates that the shape of the w/o Co II @SiO 2 and SiO 2 revealed by SAXS deviates from an ideal spherical shape. Table 3. The R g , R g *, D max , d s , FD, and V part (averaged particle volume) values, the scattering intensity at q = 0 (I 0 , in arbitrary units), and the diameters of Co II @SiO 2 and SiO 2 nanoparticles synthesized by the both methods (d TEM obtained by TEM method) are presented for the comparison. The evaluated from the SAXS data fractal dimensions (FD) of w/o Co II @SiO 2 and SiO 2 presented in Table 3 are above 3.0. Such FD values are peculiar for surficial fractality derived from the folded surface, whose density is lower than that of the silica matrix. This is in good agreement with the aforesaid assumption about the TX-100-based corona at the silica surface as schematically shown in Scheme 1. Comparative analysis of both FD and R s values reveals the insignificant difference between "empty" SiO 2 and Co II @SiO 2 w/o nanoparticles (Table 3). This similarity indicates that the distribution of the Co II ions within Co II @SiO 2 (w/o) is rather homogeneous without preferable localization within specific zones of the nanoparticles, while the lower electronic density of the exterior layer derives from the specificity of the w/o technique (Scheme 1).
The SAXS analysis of Stöber Co II @SiO 2 and SiO 2 is the peculiar case, since the greatest R g values refer to the d s -values at the level of 34-41 nm which are far below the d-values evaluated from the TEM images (Table 3, Figure S5). Moreover, more smooth profiles of the scattering curves measured for Stöber Co II @SiO 2 and SiO 2 in comparison with the w/o ones ( Figure S3) indicate greater polydispersity of the Stöber versus the w/o nanoparticles. This deviation points to inhomogeneity in electron density distribution within Stöber Co II @SiO 2 and SiO 2 , which agrees well with the zones of short-and medium-range ordering revealed from the PXRD curves of the corresponding nanoparticles ( Figure 6). The shapes of the revealed core zones also deviate from ideal spheres, which is evident from the deviation between D max иd s values (Table 3). Moreover, the FD value of the empty silica spheres made by Stöber procedure is on the level of surficial fractality (3.37), while the FD values of the Stöber Co II @SiO 2 (2.6-2.7) refer to the so-called mass fractality which indicate the inhomogeneous distribution of Co II ions within the Stöber Co II @SiO 2 . This is quite different from the homogeneous distribution within the w/o Co II @SiO 2 , where both FD and d s values of the "empty" SiO 2 and Co II @SiO 2 nanoparticles are at the same level.
In summary, the SAXS data revealed that the synthesis of Co II @SiO 2 in the framework of Stöber method results in the localization of Co II -dopant within specific zones of the silica spheres, while more homogeneous distribution of Co II -dopant within Co II @SiO 2 is obtained by the w/o technique. Moreover, the surface fractality of Co II @SiO 2 (w/o) can be explained by the porous silica surface decorated by TX-100-based corona.

Electrochemical Behavior of Co II @SiO 2
The electrochemical behavior of Co II @SiO 2 was measured with the use of carbon-paste electrode (CPE) [45]. The CPE-based technique enables avoiding effects of different colloid stability, which makes it most convenient for comparative analysis of electrochemical behavior of Co II encapsulated into Co II @SiO 2 synthesized by the Stöber and w/o methods. Two types of Co II @SiO 2 , i.e., (Stöber) and (w/o) have identical first oxidation and reduction peaks. In particular, one-electron irreversible oxidation of Co II to Co III is observed at a potential of 0.65 V (Figure 7). At −1.12-−1.15 V, a one-electron reversible reduction of Co II to Co I is observed for both types of nanoparticles, while a significant difference is observed at the second reduction of Co I to Co 0 peak. The irreversible reduction of Co I to Co 0 recorded at −1.69 V is revealed for Stöber Co II @SiO 2 , while, for w/o Co II @SiO 2 , the reduction at −1.38 V is reversible (Figure 7, Table 4). It is well-known that reduction or oxidation potentials are to a great extent dependent on the inner-sphere environment of Co II . However, the previous report reveals that predominant localization of the Ni II complexes within core zone of the Ni II -doped silica nanoparticles shifts the electrochemical Ni II to Ni I reduction to more negative potentials [15]. Thus, the second Co I to Co 0 reduction step is affected, while the Co II to Co I reduction and Co II to Co III oxidation potentials are not influenced by the synthetic method (Table 4). The results are not enough to recognize the exact reasons for the aforesaid similarities and differences in electrochemical behavior of Stöber Co II @SiO2 and w/o Co II @SiO2. Nevertheless, the electrochemical activity of the Co II @SiO2 points to their applicability in electrochemical analysis of OPC. Figure 7. CVs for w/o Co II @SiO2 (red) and Stöber Co II @SiO2 (blue) WE: CPE, H2O, 10 −1 mol·L −1 Et4NBF4 Potentials vs. Ag/AgCl recalculated to Fc + /Fc.

Co II @SiO2 for Electrochemical Determination of OPC
The presentation of an electrochemical response of Co II @SiO2 on OPC is worth preceding by the discussion of main mechanisms responsible for the response. Both oxidation and reduction of Co II centers in Co II @SiO2 are manifested in the CVs by the peaks with the peak currents dependent on   It is well-known that reduction or oxidation potentials are to a great extent dependent on the inner-sphere environment of Co II . However, the previous report reveals that predominant localization of the Ni II complexes within core zone of the Ni II -doped silica nanoparticles shifts the electrochemical Ni II to Ni I reduction to more negative potentials [15]. Thus, the second Co I to Co 0 reduction step is affected, while the Co II to Co I reduction and Co II to Co III oxidation potentials are not influenced by the synthetic method ( Table 4). The results are not enough to recognize the exact reasons for the aforesaid similarities and differences in electrochemical behavior of Stöber Co II @SiO 2 and w/o Co II @SiO 2 . Nevertheless, the electrochemical activity of the Co II @SiO 2 points to their applicability in electrochemical analysis of OPC.

Co II @SiO 2 for Electrochemical Determination of OPC
The presentation of an electrochemical response of Co II @SiO 2 on OPC is worth preceding by the discussion of main mechanisms responsible for the response. Both oxidation and reduction of Co II centers in Co II @SiO 2 are manifested in the CVs by the peaks with the peak currents dependent on concentration of the Co II centers. The increased peak current of Co II /Co III or Co II /Co I transformations results from regeneration of Co II centers due to rapid interaction of Co III or Co I with OPC. In turn, the peak current can be significantly decreased in a case of unfavorable processes (the destruction of the catalytically active form or complex formation preventing a reestablishing of Co II centers).
Both OPC (glyphosate and diethyl phosphite) are electrochemically inactive in the studied interval of potentials. Thus, the enhanced peak current at the oxidation and reduction potentials of w/o Co II @SiO 2 under the increased concentrations of the OPC (Figure 8a,c) indicates that oxidized Co III @SiO 2 or reduced Co I @SiO 2 forms return to the initial form (Co II @SiO 2 ) due to rapid redox reaction of Co III or Co I centers with the OPC. The concentration dependent increase in the peak current (Figure 8a,c) provides the electrochemical response on the OPC. The comparative analysis of the substrate-induced electrochemical response of Co II @SiO 2 synthesized by w/o and Stöber procedures reveal a significant difference between them. The difference is the highest at the reduction potential, when Stöber Co II @SiO 2 nanoparticles demonstrate a significant increase in the peak current under the growth of diethylphosphite concentration (Figure 8c), while glyphosate in the same conditions drops the peak current to zero (Figure 8d). It is worth assuming that the aforesaid fact correlates with the differences in the inner-and outer-sphere of Co II ions within the silica spheres formed by the w/o and Stöber methods. Thus, the OPC-triggered electrochemical behavior of Co II @SiO 2 is also greatly influenced by the method (Stöber or w/o) applied for their synthesis. Unfortunately, exact mechanisms responsible for the different electrochemical behavior of the hybrid nanoparticles in the presence of the OPC are not clear from the results. Additional studies lying beyond the present work scope are required for their clarification.
Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 20 concentration of the Co II centers. The increased peak current of Co II /Co III or Co II /Co I transformations results from regeneration of Co II centers due to rapid interaction of Co III or Co I with OPC. In turn, the peak current can be significantly decreased in a case of unfavorable processes (the destruction of the catalytically active form or complex formation preventing a reestablishing of Co II centers). Both OPC (glyphosate and diethyl phosphite) are electrochemically inactive in the studied interval of potentials. Thus, the enhanced peak current at the oxidation and reduction potentials of w/o Co II @SiO2 under the increased concentrations of the OPC (Figure 8a,c) indicates that oxidized Co III @SiO2 or reduced Co I @SiO2 forms return to the initial form (Co II @SiO2) due to rapid redox reaction of Co III or Co I centers with the OPC. The concentration dependent increase in the peak current ( Figure  8a,c) provides the electrochemical response on the OPC. The comparative analysis of the substrateinduced electrochemical response of Co II @SiO2 synthesized by w/o and Stöber procedures reveal a significant difference between them. The difference is the highest at the reduction potential, when Stöber Co II @SiO2 nanoparticles demonstrate a significant increase in the peak current under the growth of diethylphosphite concentration (Figure 8c), while glyphosate in the same conditions drops the peak current to zero (Figure 8d). It is worth assuming that the aforesaid fact correlates with the differences in the inner-and outer-sphere of Co II ions within the silica spheres formed by the w/o and Stöber methods. Thus, the OPC-triggered electrochemical behavior of Co II @SiO2 is also greatly influenced by the method (Stöber or w/o) applied for their synthesis. Unfortunately, exact mechanisms responsible for the different electrochemical behavior of the hybrid nanoparticles in the presence of the OPC are not clear from the results. Additional studies lying beyond the present work scope are required for their clarification.  Nevertheless, the presented results clearly demonstrate the applicability of the substrate-induced electrochemical response for quantitative analysis of the OPC. The increase in the peak current within anodic potentials range is greater than that within the cathodic range. For evaluation of a lower detection level (LOD), the peak current of Co II @SiO 2 at different concentrations of the OPC was analyzed by differential impulse voltammetry technique (Figure 9 and Figure S6). The results (Figure 9a,b) indicate that even small amounts of the OPC (0.1 µmol·L −1 ) induce the detectable peak increase; thus, this concentration level can be considered as the LOD-value. Moreover, the inset in Figure 9 indicates the linear relationship between the response currents and HP(O)(OEt) 2 (red-black) and Glyphosate (gray-blue) concentrations in the range from 0.1 to 80 µM (with a correlation coefficient of 0.99). The slope of the linear dependencies is somewhat greater for w/o Co II @SiO 2 versus Stöber Co II @SiO 2 , although the discrimination between glyphosate and diethylphosphite is rather poor for both types of Co II @SiO 2 . It is worth noting that glyphosate can be discriminated from diethylphosphite by the selective peak current increase in the cathodic range when the CPE was modified by Stöber Co II @SiO 2 .
Nanomaterials 2020, 10, x FOR PEER REVIEW 14 of 20 Nevertheless, the presented results clearly demonstrate the applicability of the substrateinduced electrochemical response for quantitative analysis of the OPC. The increase in the peak current within anodic potentials range is greater than that within the cathodic range. For evaluation of a lower detection level (LOD), the peak current of Co II @SiO2 at different concentrations of the OPC was analyzed by differential impulse voltammetry technique (Figure 9 and Figure S6). The results (Figure 9a,b) indicate that even small amounts of the OPC (0.1 μmol·L −1 ) induce the detectable peak increase; thus, this concentration level can be considered as the LOD-value. Moreover, the inset in Figure 9 indicates the linear relationship between the response currents and HP(O)(OEt)2 (red-black) and Glyphosate (gray-blue) concentrations in the range from 0.1 to 80 μM (with a correlation coefficient of 0.99). The slope of the linear dependencies is somewhat greater for w/o Co II @SiO2 versus Stöber Co II @SiO2, although the discrimination between glyphosate and diethylphosphite is rather poor for both types of Co II @SiO2. It is worth noting that glyphosate can be discriminated from diethylphosphite by the selective peak current increase in the cathodic range when the CPE was modified by Stöber Co II @SiO2. The estimated LOD = 0.1 µmol·L −1 is greater than the values reported for most sensitive sensing systems [21], but lower than the values reported for the colorimetric sensor [67]. The linearity of the peak current increase is manifested in a wider concentration range than the linearity range reported in [21] for electrochemical sensors based on Cu-BTC/g-C3N4 nanosheets (g-C3N4 = graphite phase nitrogenized carbon, BTC = benzene-1,3,5-tricarboxylic acid). Moreover, the present LOD-value is enough for evaluation of dose limit of glyphosate in a drinking water. Furthermore, the Co II @SiO2 based sensors have the characteristics of short detection time, high stability and easy operation, which is of great impact on potential applications in electrochemical OPC analysis.

Conclusions
In summary, analysis of size and content of silica nanoparticles doped by Co II ions by means of water-in-oil microemulsion (w/o) or Stöber synthetic technique confirms the applicability of the both The estimated LOD = 0.1 µmol·L −1 is greater than the values reported for most sensitive sensing systems [21], but lower than the values reported for the colorimetric sensor [67]. The linearity of the peak current increase is manifested in a wider concentration range than the linearity range reported in [21] for electrochemical sensors based on Cu-BTC/g-C 3 N 4 nanosheets (g-C3N4 = graphite phase nitrogenized carbon, BTC = benzene-1,3,5-tricarboxylic acid). Moreover, the present LOD-value is enough for evaluation of dose limit of glyphosate in a drinking water. Furthermore, the Co II @SiO 2 based sensors have the characteristics of short detection time, high stability and easy operation, which is of great impact on potential applications in electrochemical OPC analysis.

Conclusions
In summary, analysis of size and content of silica nanoparticles doped by Co II ions by means of water-in-oil microemulsion (w/o) or Stöber synthetic technique confirms the applicability of the both techniques for synthesis of the hybrid nanoparticles. Monitoring of the transformations of [Co(bpy) 3 ](BF 4 ) 2 used as the precursor for the hybrid nanoparticles through the UV-Vis spectral analysis during the synthesis highlights the decomposition of the precursor due to efficient coordination of the Co II ions via Si-Ogroups exposed at the silica seeds. Thus, the use of either [Co(bpy) 3 ](BF 4 ) 2 or CoCl 2 as the precursors results in the nanoparticles with the same inner-sphere environment of the Co II ions.
The analysis of the spectral properties in the wavelengths range of d-d transitions for the first time revealed preferable formation of tetrahedron Co II chromophores versus the octahedral ones in the the Stöber nanoparticles, while the predominance of octahedral Co II chromopheres was observed in the w/o nanoparticles. Both IR and Raman spectra indicate one more difference between the w/o and Stöber methods. The nanoparticles synthesized by the former method are decorated by the TX-100-based corona arisen from the TX-100 molecules adsorbed at the surface of the nanoparticles synthesized by the w/o method.
PXRD analysis of the synthesized nanoparticles indicated their amorphous nature, although he pracrystallinic inclusions into amorphous phase was revealed for the Stöber nanoparticles, which differentiate them from the w/o ones. The SAXS data revealed the heterogeneity of both nanoparticles, which is manifested by the presence of core zones with greater electron density than the shell zone. The sizes of the core zone are below those evaluated from the TEM images. The deviation between the sizes obtained by the different techniques for the w/o nanoparticles is no more than 20%. This enables to assume that the shell zone derives from the porous silica surface decorated by the TX-100-based corona. Both significant (above 50%) deviation revealed between the size values obtained by TEM and SAXS and specific PXRD profiles indicate the nano-structuring of the Stöber nanoparticles.
The electrochemical behavior of the Co II ions within the hybrid nanoparticles indicates the insignificant influence of the synthetic procedure at the first oxidation and reduction steps, while the second Co I to Co 0 reduction step is greatly influenced by the synthetic procedure. In particular, the reversible Co I to Co 0 reduction is facilitated within the w/o nanoparticles. The electrochemical behavior of the hybrid nanoparticles provides an efficient basis for electrochemical analysis of glyphosate and HP(O)(OEt) 2 with the lower detection limit at the level of 0.1 µmol·L −1 with the linearity of the electrochemical response in the concentration range from 0.1 to 80 µmol·L −1 , short detection time, high stability, and easy operation. The sensing properties of the hybrid nanoparticles are also influenced by the synthetic method: the electrochemical response of the Stöber nanoparticles is able to discriminate glyphosate from HP(O)(OEt) 2 , while the w/o nanoparticles are more efficient in the sensing of the both toxicants without discrimination between them.