Arsenic Detection Using Surface Plasmon Resonance Sensor With Hydrous Ferric Oxide Layer

The lethality of inorganic arsenic (As) and the threat it poses have made the development of efficient As detection systems a vital necessity. This research work demonstrates a sensing layer made of hydrous ferric oxide (Fe2H2O4) to detect As(III) and As(V) ions in a surface plasmon resonance system. The sensor conceptualizes on the strength of Fe2H2O4 to absorb As ions and the interaction of plasmon resonance towards the changes occurring on the sensing layer. Detection sensitivity values for As(III) and As(V) were 1.083 °·ppb−1 and 0.922 °·ppb−1, respectively, while the limit of detection for both ions was 0.6 ppb. These findings support the feasibility and potential of the sensor configuration towards paving future advancement in As detection systems.


Introduction
Arsenic (As) contamination in water sources remains a global issue as it continues to affect an estimate of 500 million people around the world [1].
Among the countries that have been gravely affected by As pollution include Bangladesh, Vietnam, Mexico, USA, and China [2]. High exposure to inorganic forms of As like trivalent arsenite [As(III)] and pentavalent arsenate [As(V)] has exhibited Photonic Sensors Page 2 of 8 disruptive effects on protein metabolism in general which will eventually lead to carcinogenic diseases [3]. Acknowledging the tenacity and volatility of As, the World Health Organization (WHO) has set 10 ppb as the maximum permissible limit in potable water [4].
Monitoring and detection of As within the maximum permissible limit range can be rather challenging. Ideal analytical methods not only require exceptional sensitivity, they should be able to distinguish As species in highly concentrated silicate, phosphate, and bicarbonate water matrices as well [5]. Among the go-to techniques for As detection include hydride generation atomic absorption spectrometry [6] and inductively coupled plasma [7], both noted with 0.002 ppb limit of detection. Nevertheless, these methods require bulky and expensive facilities which may not be suitable for field-testing nor accessible for under-developed countries that are battling the crisis.
Surface plasmon resonance (SPR) was reported to have successfully detected As in aqueous samples [8][9][10][11][12][13][14][15][16]. The phenomenon is induced by directing light towards a metal film through a prism which creates an oscillation of electrons on the metal-dielectric interface. At a particular incident angle, these electrons will resonate with light causing a light absorption that dynamically responds towards any changes within the external surrounding. The absorption can be quantified by analyzing the reflectivity curve of the SPR, and sample changes whether if it is due to the presence of a different compound or the occurrence of molecular binding are represented by an angle shift in the reflectivity curve. The sensing performance of SPR has shown to be enhanced by functionalizing nanomaterials.
In this study, the integration of hydrous ferric oxide, Fe 2 H 2 O 4 , as a sensing layer and its sensing performance towards As(III) and As(V) are discussed in a prism-based SPR sensor system. Fe 2 H 2 O 4 is highly regarded as a good adsorbent for As especially in As ion removal. Results incorporating Fe 2 H 2 O 4 /magnetite/reduced-graphene oxide [15] and Fe 2 H 2 O 4 /multi-walled carbon nanotube [16] composites were previously published, however, Fe 2 H 2 O 4 as a stand-alone sensing layer has yet to be reported. Both studies exhibited good sensitivity and selectivity of Fe 2 H 2 O 4 composites towards As ions which suggest Fe 2 H 2 O 4 alone could yield a similar sensing performance.

Methodology
The experimental setup used throughout this work is depicted in Fig. 1. A prism-based SPR following the Kretschmann configuration was employed by using a He-Ne laser beam at 632.8 nm operational wavelength with 1.5 mW output power as the light source. The beam was directed to an optical chopper (New Focus 3501) and a polarizer. The chopper was used to modulate the intensity of the laser output and the polarizer was employed to maximize the transverse-magnetic mode. After the polarizer, the beam was directed to a pin hole which focused the beam into a prism with refractive index of 1.778 6. The prism was stationed on a rotational platform in order to give us control of the incident angle as the laser entered the prism. The opposite surface of where the beam was internally reflected in the prism was coupled with a glass cover slide (Copens Scientific, Glaser: 22 mm  22 mm) where a layer of gold (Au) and a layer of Fe 2 H 2 O 4 were deposited onto. The deposited surface was exposed to a sample chamber where samples could be regulated and interact with the Au-Fe 2 H 2 O 4 layer. A photodetector (Thorlabs, PDA100A) was aligned to detect the reflected light from the prism. The converted signal was then amplified with a lock-in amplifier (Stanford Research SR530) before the output was retrieved and analyzed.
Au layer with a thickness of 48.3 nm was deposited onto the glass cover slide using a spin coater (Emitech K575X Turbo) [15] water, 10 ppm, 5 ppm, and 1 ppm of As(V). To test, the sample was injected into the customized cell and readings were taken 10 min after the analyte was introduced. This was to ensure that absorption of As ion onto the surface of the sensor has reached a steady state. After recording the resonance signal, the sensing surface and sample chamber were rinsed with deionized water and dried. The deposition thickness and surface roughness were physically analyzed with atomic force microscopy (AFM) analysis and theoretically validated using a multilayer expression [16]. Stock solutions of As(III) and As(V) with a concentration of 1 000 mg/mL were prepared. For As(III), 25 ml of 20% (w/v) sodium hydroxide (NaOH) was used to dilute 1.32 g of As 2 O 3 . The pH value of the solution was determined using Sartorious's pB-10 pH meter which was calibrated daily using standard solutions [19]. On the other hand, 1.542 g of As 2 O 5 was dissolved in 20% (w/v) of NaOH to make 1 000 mg/mL of As(V) as the stock solution. A systematic dilution was used to vary the concentration of the As ions; 1 ppm, 5 ppm, and 10 ppm [20]. These stock solutions were prepared daily and stored in a flask at 5 ºC in order to retain their chemical properties.

Optimization of sensing layer
The SPR response curves from the fabricated Fe 2 H 2 O 4 -SPR sensors when tested with deionized water are depicted in Fig. 2. It was observed that, higher deposition time led to a larger angle shift and increment of reflectance. When spectra obtained at 10 s and 60 s were compared, an increase in the resonance angle was observed from 54.026 5º to 54.582 6º and the reflectance was shifted from 0.288 to 0.764, respectively.

Performance of Fe 2 H 2 O 4 -SPR sensor
The performance of the fabricated Fe 2 H 2 O 4 -SPR sensor was evaluated by observing the angle shift produced in response to As(III) and As(V) at different concentrations. The SPR curves obtained from As(III) and As(V) within the concentration range of 0 ppb -10 ppb are depicted in Figs. 5(a) and 5(b), respectively. A consistent increase in angle shift was observed in response to the increment of As ion concentration. This is in accordance to the higher metal uptake as a response to the concentration tested [21]. When results obtained from both ions were compared, it is worth to note that the angle shift produced for As(V) was smaller than that produced for As(III). For example, at 10 ppb of As(V) and As(III), the attained angle shifts were 0.698 6° and 1.635 3°, respectively, showing a difference of almost 3 folds. Moreover, the correlation between the concentration and angle shift produced when tested with As(V) was not consistent. From these observations, it can be summarized that the SPR sensor was more sensitive towards As(III) than towards As(V).
With the presence of As in the sample, Fe(III) cations from Fe 2 H 2 O 4 will coordinate with the As ions, forming inner-sphere surface complexes. For As(V), a bidentate binuclear-bridging complex will form. The same occurs with As(III) with the addition of mono-dentate complexes and outer-sphere complexes where the As(III) anions bound to the hydroxyl functional groups instead, via hydrogen bond. Due to the variety of chemical pathways, the adsorption of As(III) is more favorable as compared to As(V) [22]. Figure 6 shows the SPR response when As(III) and As(V) of concentrations below 1 ppb were  introduced. The sensitivity values of the Fe 2 H 2 O 4 SPR sensor towards As(III) and As(V) were 1.083 °⋅ppb -1 and 0.922 °⋅ppb -1 , respectively. These values were retrieved from the linear trend fitting shown in Fig. 7 which indicated that the sensing layer of Fe 2 H 2 O 4 was more sensitive towards As(III) as opposed to As(V). From these findings, the lowest detectable concentrations for both ions were attained at 0.6 ppb, which was lower than given value of the highest allowable As concentration given by the Environmental Protection Agency of the European Union and USA guidelines. For concentrations lower than 0.6 ppb, angular shifts were not obtained as the spectra overlapped with the spectrum obtained for deionized water. The adsorption capacity of As on Fe 2 H 2 O 4 is dependent on the hydroxyl groups (Fe-OH) formed on the sensing layer and adsorption of As on to Fe 2 H 2 O 4 which occurs by the creation of bidentate edge-sharing and bidentate corner-sharing complexes between As and Fe-OH [23,24]. In terms of sensing performance, the Fe 2 H 2 O 4 -SPR sensor has successfully attained lower detection limit than most of the previously reported studies tabulated in Table 1. This is likely to be caused by the property of Fe 2 H 2 O 4 being highly absorptive of As ions. However, the incorporation of additional elements to Fe 2 H 2 O 4 have shown to yield better detection limit. This was the case for -Fe 2 O 3 /SnO 2 , Fe 2 H 2 O 4 /magnetite/reduced-graphene oxide and Fe 2 H 2 O 4 /multi-walled carbon nanotube with detection limits of 0.47 ppb, 0.1 ppb, and 0.2 ppb, respectively. It is presumed that the additional elements could either boost catalytic reactions or provide more active sites for the harboring of As ions which in turn heightened the sensing performance [11,15,16]. Nonetheless, the fabrication of fiber-based SPR and the addition of other elements to the composite can be tedious which gives the current proposed sensor its own leverage. Carbon nanotubes 0.1 ppb As(III) [9] Polypyrrole-chitosan-cobalt ferrite 1 ppb As(V) [10] -Fe 2 O 3 /SnO 2 0.47 ppb As(III) [11] DNA 10 ppb As(III) [12] Gold nanorods 10 ppb As(III) [13] Fe 2 H 2 O 4 -Fe 3 O 4 -rGOD 0.1 ppb As(III)/As(V) [15] Fe 2 H 2 O 4 -MWCNT 0.2 ppb As(III)/As(V) [16]

Conclusions
An SPR sensor with integrated nanomaterial for the detection of As(III) and As(V) ions was developed and demonstrated. Optimized thickness for the deposition of Au/Fe 2 H 2 O 4 sensing layer was 60 nm with sensor sensitivity values of 1.083 °⋅ppb -1 and 0.922 °⋅ppb -1 , for As(III) and As(V) within concentration range of 0.6 ppb -1 ppb, respectively. The minimum limit of detection for both tested ions was 0.6 ppb. These findings exhibit the sensing capability of the Fe 2 H 2 O 4 -SPR sensor to detect As ions. The sensing performance is at par with standard As sensing techniques and also recent reported research works.