Green Synthesis of Single-Crystalline Akaganeite Nanorods for

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Green Synthesis of Single Crystalline Akaganeite Nanorods for Peroxidase Mimic Colorimetric Sensing of Ultralow Level Vitamin B1 and Sulfide Ions Rahul Purbia, and Santanu Paria ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00390 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Nano Materials

Green Synthesis of Single Crystalline Akaganeite Nanorods for Peroxidase Mimic Colorimetric Sensing of Ultralow Level Vitamin B1 and Sulfide Ions Rahul Purbia and Santanu Paria* Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela - 769008, Orissa, India

*

To

whom correspondence

should be addressed. E–mail: [email protected] or

[email protected], Tel.: +91 661 246 2262

1 ACS Paragon Plus Environment

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Abstract Development of low cost nanomaterials for ultralow level molecular sensing applications is drawing significant attention. In this report, single-crystalline Akaganeite (β-FeOOH) nanorods have been synthesized at room temperature in aqueous medium by green protocol using tea extract for sensing applications. Microscopic investigation revealed the dimensions of nanorods are 150 ±20 nm length and 35 ±10 nm diameter. The concentration of tea extract plays a crucial role on the final shape of the nanostructure. As-synthesized β-FeOOH nanorods exhibit good peroxidase-like activity in the presence of H2O2. The synthesized βFeOOH nanorods were tested for a rapid colorimetric detection of thiamine (vitamin B1), H2O2, and S2⁻ ions in the aqueous solutions. The sensitivity of this colorimetric probe to thiamine, and S2⁻ ions was excellent with a limit of detection as low as 44 nM, and 2.19 µM, respectively. This work provides an efficient, green, reproducible, and economical approach to synthesize rod-like β-FeOOH nanostructures for sensor applications.

Keywords: Green tea, Nanorod, Thiamine, Sulfide, Peroxidase activity.


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1. Introduction Over the past decades, iron oxide nanostructures have drawn enough attention because of their unique chemical and physical properties with well-controlled shapes and sizes. Among the iron oxide family, iron (III) oxy-hydroxide including goethite (α-FeOOH), akaganéite (βFeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH) nanoparticles (NPs) have emerged as one of the most interesting nanomaterials owing to abundantly available, nontoxic, chemical stability at room temperature, resistant to corrosion, and low cost1. Because of the above mentioned physicochemical properties FeOOH nanostructures, mainly β-FeOOH nanostructures have attracted significant attention in numerous potential applications such as for the removal of pollutants22, chemical catalysis3,4, photocatalysis5–8, electrocatalysts9,10, sorption2, magnetic devices11–13, and electrode in lithium batteries14–19. Among different polymorphs of oxy-hydroxides, β-FeOOH nanostructure is most extensively studied which exhibit large tunnel-type monoclinic crystal structure framework. In the βFeOOH monoclinic structure, iron and oxygen ions occupy two, and four distinct special crystallographic positions 4i [[x,0,z]] with local symmetry m, respectively. The monoclinic structure of β-FeOOH is stabilized by chloride ion which partially present on the special crystallographic position 2a [[0, 0, 0]] with 2/m local symmetry20. In general, compared to bulk and zero-dimensional materials, one-dimensional (1D) structures such as nanorods possess unique electronic, optical, and catalytic properties in several applications because of their enrich active sites through the selective exposure of reactive facets on their surface. In recent years, β-FeOOH NPs with controlled size and shape have attracted vast attention because of their size and shape-dependent properties and potential applications. Considerable efforts have been devoted on the synthesis of 1-D β-FeOOH nanostructures using hydrolysis of iron salt7,14, chemical reduction/precipitation8,12, thermal decomposition3,15,21, microwaveassisted6,22, and hydrothermal19,23 methods. Generally, the hydrothermal method is the most 3 ACS Paragon Plus Environment


common approach for the synthesis of β-FeOOH nanorods because of controlled hydrolysis under high temperature and pressure. However, the hydrothermal techniques may have several disadvantages related to long time consumption, high temperature with careful control, and reproducibility during scale-up. In addition, many above-mentioned nonhydrothermal synthesis approaches are also required high temperature for the synthesis of βFeOOH nanorods, which can also cause the formation of Fe2O3 as impurity. Despite the intense effort for the synthesis of β-FeOOH nanorods, development of facile and sustainable synthesis protocol for synthesis of controlled β-FeOOH nanorods at room temperature is a challenging task. Till today many green synthesis processes have been recently reported for synthesis iron and iron oxide nanostructures using plant extracts as reducing or capping agent. However, green approach at room temperature for the synthesis of β-FeOOH nanorods has not been reported. As per as applications of iron oxide NPs is concern, artificial peroxidase-like enzyme activity has been increasing attention in medicine (bioanalytical and clinical), environmental, and food industries24. The enzyme-like peroxidase activity of iron oxides originates because of the presence of Fe3+/Fe2+ ions (Fenton reagent) on the surface of NPs which catalyze the decomposition of hydrogen peroxide, resulting to a colored oxidized product. Thus, various colorimetric sensing method has been developed based on peroxidase activity which are easier when compared to voltammetry, atomic absorption spectrometry, and fluorometric methods. It has also been proven that the peroxidase mimic activity of iron oxide nanorod was dominated over the spherical and other morphology25,26. While summarizing the literatures on application of β-FeOOH, it was found that there was no study available on βFeOOH nanorods for peroxidase-mimic enzymatic activity for sensing applications. This activity can also be utilized for the application in sensing of chemical or bio molecules too. Among several important bio-molecules, easy detection of thiamine (vitamin B1) is very


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important. Thiamine is an essential nutrient which plays a vital role in carbohydrate metabolism, and its deficiency causes various diseases in the human body such as beriberi, neurological disorders, and optic neuropathy, etc. Thus, developing an effective method for thiamine detection is a great interest in clinical analysis, pharmaceutical industries, and food processing. Till now, different analytical and fluorometric methods have been developed for thiamine detection using various materials such as Au27, CdSe28, CdTe29, Eu-doped Y2O330, carbon-dots31 and oxidizing agent32 (H2O2, KMnO4). Out of them, H2O2 based peroxidase method is simple, where H2O2 is used as a oxidant in the presence horseradish enzymes to make fluorescent active thiochrome formation from non-fluorescent thiamine. However, the use of horseradish enzymes will definitely increase the cost and complexity of the sensor system. In contrast, peroxidase active iron oxides materials are cheap, non-toxic, and environmentally friendly compared to the expensive horseradish enzymes. In spite of the fact that the use β-FeOOH in place of horseradish enzymes may be much better and economical, but there is no reported study till now for the ultra low level detection of thiamine by colorimetric method using peroxidase enzyme like activity of iron oxide nanostructures. In addition, it is well-known that sulfides ions are harmful contaminants which extensively released from paper, tanneries, pulp-manufacturing, food-processing, and petroleum refinery industries. Its continuous exposure at a high concentration leads to physiological and biological problems such as Alzheimer’s, Down’s syndrome, and hyperglycemia. Recently, some nanomaterials such as graphene33, and silver molybdates34 have also been reported for sulfide sensing based on peroxidase activity, but there is no study available for sulfide sensing using β-FeOOH nanorods. In this work, we have successfully synthesized 1-D β-FeOOH nanorods at room temperature by a “green” protocol in aqueous media. The green tea leaf extract was used as a reducing and stabilizing agent for the synthesis of β-FeOOH nanorods with a single 5 ACS Paragon Plus Environment


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crystalline uniform structure of 150 ±20 nm length and 35 ±10 nm width at room temperature. The as-prepared β-FeOOH nanorods have been proven to be significantly good peroxidase

activity toward

the

oxidation of

the

peroxidase

substrate

3,3′,5,5′-

tetramethylbenzidine (TMB) in the presence of H2O2. This peroxidase like activity of iron oxide NPs has been utilize for the ultralow level sensing and quantitative detection of vitamin B1 as well as sulfide ion by the simple colorimetric method. The novelty of this study is the simplicity of the β-FeOOH nanorods synthesis method with higher yield as well utilization of its peroxidase activity for easy sensing and quantitative detection of vitamin B1, and sulfide ions.

2. Experimental procedure 2.1 Materials The green tea leaves were brought from the tea garden of Darjeeling, India. The required chemicals such as anhydrous ferric chloride, anhydrous ethanol, and H2O2 were purchased from Merck and used without any further purification. All the experiments were conducted using ultrapure water of 18.2 MΩ.cm resistivity (Millipore, Elix). 2.2 Preparation of green tea extract (TE) A 10 g of fresh, dried green tea leaves were soaked in a 50 mL flask containing ultrapure water and refluxed under microwave reactor at 80 °C for 30 min. The obtained residue extract was separated by filtration through Whatman No. 42 filter paper, and subsequently, centrifuged at 15,000 rpm for 10 min in order to remove the suspended fine particles. Finally, the filtered supernatant tea extract (TE) was freeze-dried into to powder (Labconco, USA). Further, the freeze-dried powder was stored in the refrigerator for further use. Finally, 1% (w/v) solution of the extract was prepared from the freeze-dried tea extract powder to carry


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out all experiment as a reducing/stabilizing agent for iron oxide NPs synthesis. The green tea extract mainly contains different polyphenols such as, epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG); the structures of these compounds are presented in Figure S1 (Supplementary information). 2.3 Synthesis of β-FeOOH nanorods and NPs For the synthesis of β-FeOOH, various volume (0.05-1%) of green tea extracts (1% freezedried tea extract) were added in 2 ml aqueous solution containing FeCl3 (1 mM) and stirred with constant speed at room temperature. After addition of green tea extract, the formation of β-FeOOH was noticed by immediate color change. Further, the precipitate of β-FeOOH NPs was centrifuged with ethanol and water. The different TE concentrations lead to different morphology of β-FeOOH. The nanorods, mixture of spherical NPs + nanorods, and spherical NPs were synthesized using 0.05-0.40, 0.5-0.7, and more than 0.7 % TE concentrations using the above-mentioned procedure. The formation of different β-FeOOH was also observed in the form of colour change of the reaction media from pale yellow to brownish (nanorods), dark brown (mixture of nanorods and spherical), and finally, deep black (spherical) on addition of excess TE solution, respectively. The synthesized all samples were in suspension form and can be easily store at room temperature. 2.4 Characterization of NPs The phase, structure, and average crystalline size of nanostructures were measured by using a multipurpose X-ray diffraction (Rigaku, Japan/Ultima-IV) with Cu Kα radiation (1.5406Å) operating at 40 kV and 30 mA. The synthesized aqueous samples were centrifuged and dried on glass slide at 45 ºC temperature. The dried samples coated glass slide was used for powder XRD analysis. The morphologies, structures, and dimensions of the nanostructures were analyzed by using field-emission scanning electron microscope (FE-SEM, FEI, NOVA



NanoSEM) and transmission electron microscope (FEI, Technai G20 F30). UV–Vis spectroscopic measurements were recorded using an UV–Vis–NIR spectrophotometer (Shimadzu-3600). Fourier transform infrared spectroscopy (FT-IR) spectra were carried out using an FT-IR spectrometer (Thermo Fisher, Nicolet, iS-10). Fluorescence measurement was carried out using a Hitachi spectrofluorometer (F-7000) with a slit width of 5 nm for the excitation and emission.

2.5 Peroxidase-like activity and vitamin B1 sensing The peroxidase-like activity of β-FeOOH nanorods was evaluated for the oxidation of 3,3′,5,5′-Tetramethylbenzidine in the presence of H2O2. A stock solution of TMB (10 mM) was prepared in absolute ethanol. A 20 µL H2O2 (5 mM), 20 µL β-FeOOH NPs (5 mg/ml), and 80 µL (10 mM) TMB were added in acetate buffer (pH = 4) solution to make the final volume of 2 ml. Then, the UV-Vis. absorbance spectra at 653 nm related to oxidized TMB were measured after 10 min of incubation time. Corresponding control was also done without adding β-FeOOH nanorods. Further for H2O2 sensing, the different concentration of H2O2 was added to the above solution and incubated for 10 min. Then, the absorbance spectra at 653 nm related to oxidized TMB were measured. For vitamin B1 sensing, a 2 ml of acetate buffer (pH = 4) solution containing 20 µL TMB, 20 µL β β-FeOOH nanorods were incubated for 10 min at room temperature. Then, different concentrations thiamine was added to the above solution, and incubated for 10 min. Finally, 15 µL H2O2 was added in the aboveprepared mixture and the absorbance of blue color solution of oxidized TMB was measured by UV-Vis spectroscopy.

2.6 Colorimetric detection of Sulfides using β -FeOOH nanorods


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The peroxidase activity of β-FeOOH nanorods was used for the detection of sulfides ions. A 2 ml of acetate buffer (pH = 4) solution containing 20 µL TMB, 15 µL H2O2 and 20 µL β FeOOH nanorods were incubated for 10 min at room temperature and used for sulfide ions detection. A 20 µL aqueous solution of different concentrations sulfide ions (Na2S) was added to the above solution, and the absorbance of blue color solution of oxidized TMB was measured by UV-vis spectroscopy. To test the selectivity, other anions (F⁻ , Cl⁻ , Br⁻ , NO3⁻ , NO3⁻ , S2O3⁻ , SO4⁻ , and PO43⁻ ) were also added in the mentioned solution (10 times higher concentration than the sulfide ion concentration) and analyzed by UV-Vis spectroscopy. 3. Results and Discussion 3.1 Synthesis process In this study, β-FeOOH NPs was synthesized at room temperature by a simple reaction between FeCl3 and TE. The presence and absence of oxygen during the reaction plays a significant role in this synthesis process. To see the effect, initially synthesis was done in the presence and absence of oxygen and followed by XRD analysis of the obtained NPs. From the analysis it can be seen that well crystalline β-FeOOH NPs was obtained in the presence of oxygen (Figure 1(a)), whereas, the reaction in inert atmosphere produced the amorphous Fe(OH)2 NPs (Figure 1(b)). The physical color appearances after the reaction also indicate the products are different in two cases. The addition of TE to the aqueous solution of FeCl3 in the presence of oxygen results in immediate color change from yellow to brownish (ferric ions), while that in inert condition to green (ferrous ions). Based on the above observation, it can be concluded that the formation of β-FeOOH takes place mostly in three successive steps: (i) hydrolysis of FeCl3 to Fe(OH)3, (ii) reduction of ferric iron (Fe(OH)3) to ferrous iron (Fe(OH)2) by the tea polyphenols (the reduction potential of green tea polyphenols are in the range of the 0.3-0.8 V, which can easily reduce the Fe3+ (-0.44 V vs. SCE)35,36), (iii) rapid 9 ACS Paragon Plus Environment


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oxidation of the Fe(OH)2 oxidize into crystalline iron oxyhydroxides in the presence of dissolved oxygen present in water37,38. Further, to support the reduction mechanism by TE, the FT-IR analyses of TE before and after reaction, and formed β-FeOOH NPs were carried out and presented in Figure 1 (c). –1

From Figure 1 (c), it can be observed that unreacted TE has a strong band at 1,698 cm –1

corresponding to the –C=O stretch in the aromatic ring. The band at 1,607 and 1457 cm is corresponding to –C=C, and –C–C stretching of in-ring aromatic groups, respectively. The –1

band at 1,741 cm is corresponding to C–O–C stretching of polysaccharide molecules. The peak at 1,370 cm−1 could be assigned to the –OH stretching vibration of phenolic group. The peaks at 1,238 and 1047-1,080 cm−1 could be attributed to the –C–O and –CH stretching vibrations of the polyols compounds, respectively. Thus from the IR spectr, it can be observed that unreacted TE sample is rich in polyphenols, and polysaccharides39,40. After reduction of Fe3+, the dominant peak of TE was suppressed in the fingerprint region (10001700 cm−1) because of participation of phenolic compounds in the reduction reaction as shown in Figure 1(c). Finally, the FT-IR spectra of water washed β-FeOOH was investigated and shown in Figure 1(c). It shows a broad peak at 3416 cm-1 because of the stretching vibration of O–H in β-FeOOH. The weak vibration at 2800-2900 cm-1 and 1645 cm-1 was found because of the C–H and –C=O stretching vibration of aromatic rings of polyphenol capping on NPs, respectively. The band located at approximately 1016 cm-1 is because of the weaker hydrogen bonds between hydroxyl groups of the β-FeOOH structure. The characteristics peaks at wavelengths 680 and 845 cm-1 are assigned to the vibration modes of the Fe–O coordination.


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Figure 1 (a) XRD pattern of β-FeOOH nanorods and NPs obtained after reaction in the presence of oxygen, (b) XRD spectra of as-synthesized amorphous Fe(OH)2 particles obtained after reaction in inert atmosphere, (c) FT-IR spectra of tea extract before and after reaction, and formed β-FeOOH NPs 3.2 Structural and morphological characterization To confirm the phases of synthesized iron oxide (β-FeOOH), the powder XRD patterns of the as-synthesized samples were investigated. As displayed in Figure 1(a), the obtained XRD peaks at 27.5, 33.8, 35.10, 39.28, 46.55, 55.9, and 56.04° are indexed to {310}, {400}, {211}, {301}, {411}, {521}, and {541} crystallographic planes of monoclinic phase of the βFeOOH structure, respectively, corresponding to JCPDS file no. 42-1315. Since all diffraction peaks are sharp and strong, indicate good crystalline nature of as-synthesized βFeOOH. While analyzing the peak intensities, it has been found that the intensity of the {211} peak is higher than that of the {310}, this imply that the β-FeOOH is mainly present as rod-like crystals1,11,41. The higher intensity diffraction peak of {211} suggests dominant crystal growth along the {001} direction for β-FeOOH nanorods. The XRD analysis of βFeOOH spherical NPs exhibits the similar diffraction peaks (JCPDS no. 42-1315) as shown in Figure 1(a), but the diffraction peak intensity of {211} is lower compared to the {310} peak, implies the difference in crystal orientation between nanorods and NPs.



Figure 2 (a) FE-SEM images of β-FeOOH nanorods, (b-c) TEM images of β-FeOOH nanorods, (d) HR-TEM image of β-FeOOH with lattice fringes, inset of d corresponding (e) FFT pattern (f) SAED ring pattern of β-FeOOH. The shape and morphological characteristics of the synthesized β-FeOOH were analyzed using FE-SEM (field-emission scanning electron microscope) and TEM (transmission electron microscope). It can be clearly seen from Figure 2(a) that the length and diameter of rod-like morphology of β-FeOOH are 150 ±20 nm and 35 ±10 nm, respectively. The FE-SEM images also clearly show the obtained β-FeOOH nanorods are dispersed uniformly without any agglomeration. From the TEM micrograph of β-FeOOH, it can be seen that well-defined structure of β-FeOOH nanorod with similar dimension to that FE-SEM analysis (Figure 2(b-c)). The magnified TEM image (Figure 2c) shows a visible thin organic layer (4.6 ± 1 nm thickness) on the surface of the nanorods, because of the 12 ACS Paragon Plus Environment

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adsorbed polyphenols molecules, which improves the colloidal as well as further oxidation stabilities of iron oxyhydroxides (β-FeOOH). To get the information about lattice arrangement of β-FeOOH nanorods, the HR-TEM analysis was done and illustrated in Figure 2(d). It clearly indicates continuous parallel lattice fringes with uniform contrast in a single nanorod, reveals its single crystalline nature at room temperature. The measured lattice spacing was 0.25 nm, matching with the d-spacing of {211} planes of β-FeOOH, which are in good agreement with the XRD analysis. This analysis also indicates dominant crystal growth along the (001) direction for nanorods and preferentially exposed (211) planes to surface with high iron density which might be good for catalytic activity. Further, the monoclinic-like spot arrays (Figure 2e) shown by the fast Fourier transform (FFT) of single β-FeOOH nanorods (Inset of Figure 2d) verifies the single-crystal nature of the nanorods and monoclinic crystal structure of the β-FeOOH as confirmed by XRD and HR-TEM analysis. The selected area electron diffraction (SAED) pattern of β-FeOOH nanorods exhibits dot pattern corresponding to single crystalline nature of monoclinic β-FeOOH nanorods as shown in inset of Figure 2(f). While the SAED pattern of β-FeOOH spherical NPs exhibit many diffraction pattern corresponding to polycrystalline nature as shown in Figure S2 (Supplementary information).

3.3 Growth and formation mechanism The reduction of FeCl3 with tea extract and subsequent oxidation step for the formation of βFeOOH nanorods is very fast at room temperature, as a result it is difficult to analyze through time-dependent study. Thus, to elucidate and understand the growth process of β-FeOOH nanorods, the samples obtained after immediate (~1 min) addition of FeCl3 with TE were analyzed with FE-SEM and TEM. It can be observed from FE-SEM images (Figure 3(a)) that there are irregular, and very small size pseudo-spherical NPs and nanorods are formed at 13 ACS Paragon Plus Environment


the initial period. The TEM images of initial samples (Figure 3(b-c)) are also revealed particular two types of crystalline morphology including pseudo-spherical (irregular shape) particles with 4 ±2 nm size (yellow circled in Figure 3 c), and small nanorods with 20 ±2 × 2 ±0.5 nm (red circled in Figure 3 c). A closer look at the image shows pseudo-spherical particles approach to a closer distance and forming a self-assembled structure, which in turn leads to a nanorod morphology. The lattice fringes (d211 = 0.25 nm) of spherical NPs obtain from the initial stage in Figure 3(c) indicate the single crystalline feature. Based on the above experimental observations, it can be proposed that the rapid reaction would yield numerous β-FeOOH nuclei, which then converted to pseudo-spherical seed NPs at the initial period, and finally organized into a single crystalline rod-like nanostructures to minimize the surface energy. When the reaction duration is prolonged to 5-10 min (Figure 2), the product contains a large number of uniform size nanorods. After that the size and morphology was not changing even after 10 h.

Figure 3 Time-dependent growth experiments of β-FeOOH samples obtained after immediate (~1 min) addition of FeCl3 with tea extract, (a) FE-SEM images, and (b-c) TEM images. Further, to understand the effect of different TE concentration in the presence of constant FeCl3 (1 mM) concentration, a series of the experiments were carried out by varying the TE volume (0.05 to 1%) in the reaction system, and corresponding morphology was 14 ACS Paragon Plus Environment

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microscopically monitored by FE-SEM and TEM as shown in Figure 4. Interestingly, it was found that the change in TE concentrations lead to different morphology of β-FeOOH products. Upon addition of low TE concentration (0.05 to 0.40%) in the FeCl3 solution (Figure 4(a-b)), β-FeOOH nanorods with uniform shape was obtained. By increasing the TE concentration (0.5 to 0.7 %), mixed morphology of spherical and rod-like nanostructures were obtained as seen in Figure 4 (c-d). However, only agglomerated type network of βFeOOH spherical NPs were observed upon addition of more than 0.7 % TE concentration (Figure 4(e-f)). The sequential formation of β-FeOOH NPs was also observed in the form of colour change of the reaction media from pale yellow to brownish (nanorods), dark brown (mixture of nanorods and spherical), and finally, deep black (spherical) on addition of excess TE solution, respectively, as shown in Figure S3 (Supplementary information). The XRD patterns of three different β-FeOOH morphology obtained by varying concentrations of TE also indicate the presence of single phase pure β-FeOOH pattern (Figure 1(b)), which confirms that the concentration variation of TE did not affect the crystal phase of the product.



Figure 4 Tea extract concentration-dependent growth of β-FeOOH nanorods. FE-SEM and TEM images of β-FeOOH at different ratio of tea extract solution in FeCl3 solution (a-b) 0.05-0.40%, (c-d) 0.40-0.70%, (e-f) more than 0.7%. From the above results it is evident that the shape transformation of β-FeOOH is mainly driven by TE concentration. After summarizing TE concentration based microscopic results, the possible formation mechanism of β-FeOOH nanorod has been illustrated in Scheme 1. In this process, after addition of FeCl3 to TE very fast reactions such as hydrolysis, reduction, and oxidation occur successively in the presence of tea polyphenols and developed enough nucleation sites for the formation of β-FeOOH NPs. When the polyphenol/TE concentration is low, the polyphenols molecules are preferably and sparsely adsorbing on the {211} and other high energy facets of the seed particles. As the surface 16 ACS Paragon Plus Environment

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energy of newly formed nuclei are high, they are thermodynamically unstable. Under this condition, in the presence of low TE concentration, sparsely adsorbed polyphenols are unable to give the thermodynamic stability of seed β-FeOOH NPs; however, they tend to reduce their surface free energy through the growth process in a certain direction. During the selective adsorption of polyphenols on the {211} and other high energy facets of β-FeOOH, the {100} facet of the seed particles have preferable longitudinal z-axis [001] direction growth to form nanorods. However, in case of excess TE, the chances of adsorption of polyphenol molecules on all the energy facets of β-FeOOH NPs because of proper availability of excess polyphenols is more and finally gives a spherical shape stable NPs. For the reproducibility purpose, we synthesized the nanorods more than 10 times using this method and found the similar results.

Scheme 1. Proposed formation mechanism of β-FeOOH nanorods and spherical NPs in the presence of different amount of tea extract (a) low polyphenols capped β-FeOOH seed particles, (b) growth of the seed particles towards [001] direction to form nanorod, (c) TEM image of β-FeOOH aggregation at the initial stage, (d) polyphenol capped β-FeOOH nanorods, (e) TEM image of polyphenol capped nanorods, (f) spherical β-FeOOH nanoparticles in the presence of excess polyphenols, (g) TEM image of β-FeOOH under excess TE. 17 ACS Paragon Plus Environment


3.4 Peroxidase-like activity Iron oxides NPs have been widely used for the peroxidase-like activity. In this study, the catalytic efficiency of as-synthesized β-FeOOH nanorods was investigated by oxidation of peroxidase substrate 3,3′,5,5′-Tetramethylbenzidine (TMB) to the blue colored oxidized product 3,3′,5,5′-Tetramethylbenzidine diimine (TMBDI) in the presence of H2O2 at room temperature which can be traced by the absorbance maxima at 652 nm. The peroxidase enzyme-like activity of iron oxides originates because of the presence of Fe3+/Fe2+ ions (Fenton reagent) at the NPs surface which is known to catalyze the decomposition of hydrogen peroxide, resulting to a colored product as shown in equations 1-3. The TMB is commonly used for immunoassays and the evaluation of NPs peroxidase-like activity. It is also well known from the literature that the peroxidase-like catalytic activity was much higher in acidic media (pH 4–5) than in neutral and basic media. Thus, the catalytic activity of β-FeOOH was investigated at pH 4.0. As shown in Figure 5(a-b), the β-FeOOH nanorods can act as a catalyst for the quick oxidation of peroxidase substrate TMB in the presence of H2O2 which was confirmed by a sudden color change from transparent to blue color. Control reaction was performed in the absence of β-FeOOH nanorods which showed no color change in the same period reaction as illustrated in Figure 5(b). The colorimetric changes were also detected by UV-Vis. spectral analysis. In the absence of β-FeOOH, the spectra of TMB and TMB + H2O2 remained unchanged. Upon the addition of β-FeOOH nanorods, new absorption peaks at 652 nm, and 370 nm for oxidized TMB were observed. So the formation of oxidized TMB indicates that β-FeOOH NPs exhibit peroxidase-like activity similar to nature of HRP (horseradish peroxidase enzyme). Further to exclude the possibility of the peroxidase-like activity resulted from the leaching of free Fe2+/Fe3+ ions in the suspension, the β-FeOOH NPs were incubated in the buffer solution for 10 min. The solution was centrifuged, and the 18 ACS Paragon Plus Environment

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supernatant was tested for peroxidase activity with H2O2 and TMB. No such activity in the supernatant indicates the peroxidase-like activity is solely for the β-FeOOH nanorods and not from the leached metal ions as shown in Figure S3 (Supplementary Information).

To study the structural advantage of nanorod shape, we compared the catalytic activity with spherical NPs. Figure S5 (Supplementary information) indicates the UV-Vis. spectrum of nanorods has the largest absorption peak at 652 nm, while that of spherical NPs show less catalytic activity. The rod-like structure showed 2.5 times higher activity compared to spherical NPs. In addition, for both cases physical appearance can also be visually observed through the color change as illustrated in Figure S5 (Supplementary information). The reasons for the enhanced catalytic activity of nanorods can be attributed as the presence of high atomic density exposed on (211) planes. The high surface area of nanorods with more active surface sites for the catalytic reaction compared to spherical NPs and presence of distinct particles without agglomeration. Finally, the higher affinity of TMB molecules towards higher negatively charged β-FeOOH nanorods (zeta potential = - 24.5 ±0.2 mV measured experimentally) compared to that of spherical NPs (- 14.9 ±0.3 mV). In this catalytic process, the TMB oxidation reaction strongly depends on the concentration of H2O2. It is noteworthy to mention that the H2O2 detection has significant importance in many fields such as biotechnology, medicine, environmental protection, and food industries. As a result, peroxidase-like activity of β-FeOOH was used for the detection 19 ACS Paragon Plus Environment


of H2O2. The UV-Vis. spectra of TMB in the presence of different H2O2 concentration shows that the absorbance of TMB increases with the increase of H2O2 concentration as shown in Figure 5(c). It can be seen in Figure 5(d) that the standard H2O2 concentration response curve is linear in the range of 100 to 500 µM concentration.

Figure 5 (a) The UV-Vis. Spectra of TMB, TMB + H2O2, and TMB + β-FeOOH + H2O2, (b) the color change of these systems, (c) UV-Vis. spectra for the mixed solutions of TMB, βFeOOH nanorods and various concentrations of H2O2 in acetate buffer (pH 4.0) 20 min, (d) linear calibration plot of H2O2 sensing using β-FeOOH nanorods. 3.5 Colorimetric sensing for thiamine The peroxidase like activity can be utilised for several applications, however, we utilized this activity here for the ultralow level colorimetric detection of thiamine. In this method, TMB was used as a colorimetric indicator for the spectrometric detection of thiamine in the aqueous solution. The absorbance spectra of TMB + β-FeOOH nanorods + H2O2 in the presence of different thiamine concentrations (34–270 µg/L) show that the absorbance 20 ACS Paragon Plus Environment

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intensity of the mixture decreases with the increase of thiamine concentration as presented in Figure 6(a). The physical appearances of gradual decreasing color intensity of suspension mixture with the increase in thiamine concentration can be seen in inset of Figure 6(a). From the absorbance intensities, a linear standard calibration curve was obtained between ∆A (A0A) and concentration of thiamine as presented in Figure 6(b) with correlation coefficient of > 0.99. Where A0 and A are absorbance intensities of the final mixture (TMB + β-FeOOH nanorods + H2O2) in the absence and presence of thiamine, respectively. The limit of detection (LOD) was calculated from 3σ/m, where 3 is used for signal to noise ratio, σ is the standard deviation of blank measurement and m is the slope of the calibration curve. The calculated limit of detection is found to be 44 nM, which is significantly lower or comparable to the previously reported detection methods as shown in Table S1 (Supporting Information). To check the accuracy of the LOD value, an aqueous solution of 59 nM thiamine concentration was tested which is close to the LOD value. Here, we obtained an A0–A absorbance value of 0.084 from the linear calibration plot and 0.081 directly from the instrument. So, the error for this concentration is 3.57%. In order to evaluate the selectivity of the proposed detection method of thiamine, the absorbance response was measured in the presence of urea, starch, glucose, and different inorganic ions (Ca2+, Mg2+, Na+, K+, Cl-, NO3-). As shown in Figure 6(c), the detection efficiency of thiamine is significantly much higher than other molecules. The above results clearly demonstrate that this sensing method is highly selective towards the thiamine molecule and does not suffer from interference by other molecules and ions. Further, to validate the applicability of thiamine biosensor for real samples, we tried to determine the thiamine concentration in a commercial vitamin B-100 tablet which contains thiamine, vitamin B2 (riboflavin), vitamin B6 (pyridoxine HCI), vitamin B12 (cyanocobalamin), niacin, calcium pantothenate, biotin, and folic acid. The accuracy of the detection method



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was investigated by performing recovery tests on standard additions method. The tablet aqueous solution was diluted to a suitable concentration for our proposed detection assay method. The thiamine quantity of the commercial tablet was experimentally calculated to be 1.976 ±0.008 mg with respect to 2 mg original value and found error was 1.2% by the colorimetric analysis method. Although, the amounts of thiamine obtained by this method were slightly lower than that claimed by the manufacturers, they were still reasonably in agreement with the nominal values. These results clearly indicated that the present method is reliable and practically applicable.

(4) In this detection process, when both thiamine and TMB are present in the nanorod suspension, the formed hydroxyl radial first oxidize thiamin to thiochrome as shown in Equation 4 and then rest is utilized for the oxidation of TMB to get colored compound for quantification. Thiochrome is a fluorescent active compound gives characteristic fluorescence emission spectrum at 425 nm (λmax) when excited at 368 nm wavelength. In the present study, we found that the characteristic peak intensity of thiochrome is increasing linearly with the increasing concentration of thiamine in the mixture from 33 – 220 µg/L as shown in Figure 6(d). Further, to check the visual difference, the images of thiamine samples before and after peroxidase reaction were taken in the presence of 390 nm excitation wavelength light source and shown in inset of Figure 6(d). The images clearly show that without peroxidase activity fluorescence emission is absent but after peroxidase activity it shows blue color fluorescence because of thiochrome formation. Noteworthy to mention that the detection is also possible in this process by the fluorometric method, however, as colorimetric method is much simple we highlighted here. 22 ACS Paragon Plus Environment

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Figure 6 (a) Absorbance spectra of TMB after oxidation in the presence of various concentration of thiamine, (b) linear calibration plot of thiamine concentration vs. absorbance in the presence β-FeOOH nanorods, (c) selectivity of β-FeOOH catalyst towards various anions, cations and molecules, (d) The linear increase in the fluorescence intensity at 425 nm with the increase in thiamine concentration in the presence of β-FeOOH+H2O2+TMB, inset images show visual difference of thiamine samples before and after peroxidase reaction when excited at 390 nm wavelength.



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3.6 Colorimetric sensing for sulfide ion detection The excellent peroxidase-like activity of β-FeOOH nanorods can also be utilized for the spectrometric sensing of low concentration sulfide ions based on the same principle as discussed before. In this method, the absorbance of UV-Vis spectra of TMB + β-FeOOH nanorods + H2O2 in the presence of sulfide ions decreases linearly with the increase in concentration from 5-30 µM as shown in Figure 7(a-b). The LOD was also calculated and found to be 2.19 µM which was much lower than the maximum level of S2− ions (15 µM) in drinking water permitted by the World Health Organization (WHO). The limit is significantly lower or comparable than the values of other reported in the literature as seen in Table S2 (Supplementary information). The high sensitivity of β-FeOOH nanorods is mainly because of the anisotropic morphology. In order to evaluate the selectivity of the proposed colorimetric detection method of sulfide, the absorbance spectra were investigated as a control in the presence of 7 anions including F⁻ , Cl⁻ , Br⁻ , NO3⁻ , NO2⁻ , S2O3⁻ , SO42-, and PO43⁻ . As shown in Figure 7(c), the suppression of TMB oxidation is significantly higher in the presence of sulfides ions compared to the other anions, which can be clearly seen from the color of the solutions (Figure 7(c)). These selectivity results support the suitability of our proposed method for the quantification of sulfide ions in real water samples. The catalytic oxidation of TMB inhibits in the presence of sulfide, which in turn also inhibits the formation of blue colored oxidized compound of TMB, as a result, there is a decrease in the absorbance at 652 nm as seen in Figure 7(a). In the presence of sulphide ion, S2- may react with H2O2 to produced either elemental sulphur or sulphate as shown in Equations 5-6. As a result, the generation of •OH radicals by Fenton reaction is less to oxidize TMB because of competitive consumption of H2O2 by S2-ions. HS‾ + H2O2 + H+→ S0 + 2H2O

(acidic)


(5)

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HS‾ + H2O2 → SO42- + 4H2O + H+

(acidic)

(6)

Finally, the XRD and TEM analyses were done after peroxidase reaction. From the XRD spectra, the phases were found to be similar to the fresh β-FeOOH sample (Figure S6 (a), Supplementary information). However, form TEM analysis, it was found that some rods were deformed into some small β-FeOOH NPs (Figure S6 (b-c), Supplementary information). Thus, we can conclude that the samples can be reused again but the activity may reduce. For the comparison purpose, the size distribution of the nanorods obtained by this method was also compared by other synthesis routes in the literature as presented in Table S3 (Supplementary information). Finally, it was found that our preparation method was easy to synthesized uniform nanorods. The as- β-FeOOH nanorods were exclusively and succesfully tested for a rapid colorimetric detection of thiamine (vitamin B1), H2O2, and S2⁻ ions in the aqueous solutions. Furthermore, detection of these ions or molecules have huge practical importance in several industries. In fact, this technique can be used for the development of portable devices for instant detection. As a continuation of this work we are trying to detect the concentration by color intensity based image processing technique without any instrumental technique.



Figure 7 (a) absorbance spectrum of TMB oxidation in the presence of various concentration of sulfide ions, (b) linear calibration plot of sulfide sensing using β-FeOOH nanorods, (c) selectivity of β-FeOOH catalyst towards various anions including F⁻ , Cl⁻ , Br⁻ , NO3⁻ , S2O3⁻ , SO4⁻ , and PO43⁻ .

4. Conclusions The anisotropic β-FeOOH nanorods with the length of 150 ±20 nm length and 35 ±10 nm thickness were synthesized at room temperature using green tea extracts as reducing as well as capping agent. An optimum concentration of tea extract plays a crucial role on the formation of rod-like structure. This approach for β-FeOOH nanorod synthesis is reproducible and economical. The selective adsorption of polyphenol molecules from tea extract on the (211) plane of β-FeOOH leads to unidirectional growth and finally formation of rod-like structure. The obtained β-FeOOH nanorods have shown very good peroxidaselike activity for the oxidation of TMB in the presence of H2O2. The catalytic activity of the βFeOOH was shape dependent, and the rod-like structure showed 2.5 times higher activity compared to spherical NPs. The peroxidase-like activity of this material was also utilized for the detection of thiamine and sulfides ions and the limit of detections were found to be 44 nM, and 2.19 µM, respectively. These limits are much better compared to the previous reported studies, along with the simplicity of the method. The selectivity tests also show that several inorganic and organic molecules do not interfere on the analysis.

Supporting Information Available: The Supporting Information included additional figures as mentioned in the main text (Chemical structures of green tea polyphenols, SAED pattern of β-FeOOH spherical NPs. the


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color observation of β-FeOOH NPs at differnt TE solution, leaching test for peroxidase-like activity of the β-FeOOH nanorods, structural advantage of nanorod shape over spherical NPs, structural analysis of β-FeOOH sample after peroxidase activity, and Comparative tables with other reported studies). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments The financial support from the DST-SERB (ref. No. EMR/2016/000810) for this project is gratefully acknowledged. The authors also acknowledge Mr. Subhabrata Chakraborty, NIT Rourkela, for helping in FE-SEM and HR-TEM characterizations.

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Graphical Abstract


Green Synthesis of Single-Crystalline Akaganeite Nanorods for

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