Porous, pH Responsive and Reusable Hydrogel Beads of Bovine

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Energy, Environmental, and Catalysis Applications

Porous, pH Responsive and Reusable Hydrogel Beads of Bovine Serum Albumin_Au Hybrid as Smart Nano Factories for the Removal of Organic and Inorganic Pollutants from Water: A Detailed Demonstration by Spectroscopy and Microscopy Aekta Upadhyay, and Chebrolu Pulla Rao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20027 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Porous, pH Responsive and Reusable Hydrogel Beads of Bovine Serum Albumin_Au Hybrid as Smart Nano Factories for the Removal of Organic and Inorganic Pollutants from Water: A Detailed Demonstration by Spectroscopy and Microscopy

Aekta Upadhyay and Chebrolu Pulla Rao* Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India, E-mail: [email protected]

ABSTRACT The availability of potable water is one of the major concerns in many countries today. This brings in a need to design molecular scaffolds suitable for efficient adsorption of the contaminants present in water. In this paper, an unprecedented strategy is demonstrated in order to synthesize highly porous bovine serum albumin_Au (BSA_Au) beads and were employed for the removal of water contaminants. The beads stored in the acidic medium (Beada) and in the basic medium (Beadb) selectively adsorbs anionic and cationic species that includes organic dyes and inorganic species respectively by showing pH responsive behaviour. This phenomenon bestowed the beads with the recyclability and reusability as demonstrated for eight cycles where in almost 100% efficiency is retained by the beads without any deterioration.

The porous nature of these beads is retained even after switching

the pH for several cycles of adsorption-desorption processes as judged based on the SEM data. Thus, the reported beads are demonstrated for their selective and efficient removal of both organic and inorganic contaminants from water wherein the beads can be recycled by triggering pH changes which would also release the captured species. The biologically benign beads act as reusable smart nano factories in the purification of water fromindustrial contaminants. ------------------------------------------------------------------------------------------------------------Keywords:

BSA_Au hydrogel beads, pH responsive, recyclability, porous structure,

microstructural reversibility, selective adsorption, organic dyes, toxic inorganic species

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INTRODUCTION In the industrialized era, the water is contaminated by a variety of impurities, and hence easy, efficient and economically viable means of purification of water poses challenges to both scientists and technologists.1,2 It is therefore, important to develop sustainable materials and methods to remove contaminants, such as, organic dyes and toxic inorganic cationic and anionic species from water and be able to reuse the same for multiple cycles.3,4,5 Owing to their porous structure as well as their chemical and charge nature, the hydrogels would provide suitable platform for easy and efficient removal of contaminants from water.6-12 The hydrogels can be tuned to specific sizes and shapes including in the form of beads which will have their inherent properties and can serve specific applications including ion and molecular recognition,13,14 drug delivery,15-18 tissue engineering19 and encapsulation of proteins.20,21,22 The hydrogels will bring additional advantages when these are prepared from biological resources, such as, proteins in terms of their availability, biocompatibility, reusability and sustainability. Thus, this paper deals with the use of a model protein, bovine serum albumin (BSA) which is relatively cheap and abundant and its

gold coated

nanoclusters (Au_NCs) are known to exhibit aggregation depending upon the pH of the medium.23,24,25 Taking the advantage of these factors, we have developed a methodology to prepare beads which can be handled with ease. The beads were thoroughly characterised by microscopy and spectroscopy in order to explore their inherent properties. The beads are stable in acidic as well as in basic medium and interestingly the nature of their surface charge is modified which turns out to be boon-in-disguise in the removal of both anionic as well as cationic species (immaterial of whether these are organic or inorganic) with high adsorption capacity and selectivity. All these features were demonstrated by spectroscopy and microscopy by carrying out detailed array of experiments.

Thus, the porous and pH

responsive BSA_Au hydrogel beads reported in this paper are recyclable to act as efficient


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nano-factories for the purification of water from a variety of organic and inorganic contaminants by going through smart chemical events for which the details were established as reported in this paper.

EXPERIMENTAL BSA, HAuCl4, metal perchlorate salts and all the dyes were procured from Sigma Aldrich Co., and all the experiments were carried out using Milli Q water. The instruments used for the characterization are Cary 10 Bio UV-Visible spectrophotometer for the absorption, Varian Cary Fluorescence Spectrometer (λex = 500 nm) for emission, Perkin Elmer spectrometer (Spectrum One) for FT-IR, Axis Supra (Kartos Analytical) for XPS, Diamond TG/DTA (Perkin Elmer) for TGA, ARCOS, Simultaneous ICP Spectrophotometer for atomic composition, TECHNAI G2, F30, Transmission electron microscopy (by drop casting a dilute solution on a 300 mesh copper grid) and JSM-7600F Field Emission Gun Scanning Electron Microscope (FEG-SEM) in cryo mode (by freezing the sample under liq. N2 and then fracturing and sputtering for 30 sec with 10 mA current before analysis). Synthesis and Characterisation of BSA_Au NCs. The BSA_Au NCs were synthesized as per the reported procedure,26,27 by appropriately introducing changes in the concentration of BSA and HAuCl4 as given in SI 01. The BSA_AuNCs were characterized by both spectroscopy and microscopy techniques. Synthesis and Characterization of BSA_Au Beads. One ml solution of BSA_Au NCs was mixed with 60 µl of glutaraldehyde (25%) and 40 µl of glycerol (85%) and loaded in a pipette and dropped into 5% of CH3COOH from a distance of ~3 cm such that the shape of the drop is maintained and homogenous beads (Beada) are formed. These beads are transferred with the help of spatula to 0.1 M NaOH to give basic beads (Beadsb). Both the



beads were stored separately. The swelling property of Beada in 0.1 M NaOH to result in Beadb was measured by capturing an image by a digital camera and the diameter of several beads were measured using ImageJ and averaged. Similarly, the average weight of the beads in acidic as well as alkaline media was measured by analytical balance after wiping out the water with tissue. The weight % of water content present in Beada and Beadb were also quantified using TGA in which the sample was heated under N2 atmosphere from 25°C to 600°C. The samples of Beada and Beadb were analysed by FEG-SEM. The Beadsa was transferred to basic medium to convert it to beadb and again back in acidic medium to reconvert back to Beada, i.e., Beada  Beadb back to Beada. Cryo SEM was checked at all the three stages in order to ensure the microstructural reversibility of the beads. The secondary structural changes in BSA_Au NCs and beads were analysed by FT-IR spectroscopy by measuring 36 scans and averaging the data. The lyophilised Beada was imaged under SEM and mapped for elements in order to confirm the presence of Au. The lyophilised sample of the bead was physically damaged and dispersed in water under sonication to analyse by TEM. The oxidation state of Au in Beada was also confirmed by XPS. Specific Adsorption of Anionic Dyes by Beada. In 2 mL vial, 100 mg/L of anionic dye, such as, Coomassie blue brilliant R 250 (CBB), bromophenol blue (BB) or eosin yellow (EY) was taken and 1 bead (dry weight of 0.8 mg) was incubated in case of each of the dye. After specific intervals of time, 100 µl of supernatant dye solution was taken out and UV-Visible absorption spectra were recorded from 200 to 800 nm. The adsorption capacity (q) of bead for each of the dye was derived as per the details given in the supporting information28 (SI 02).


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In order to understand the adsorption mechanism of dyes by the beads, the data of q versus time t was fitted to pseudo first order as well as pseudo second order using the formulae given in SI 02. The adsorption data was fitted to both Langmuir and Freundlich adsorption isotherm models. The details for these are given in SI 02. In order to study the competitive adsorption of specific dye from a mixture, one mL of each of 50 mg/L of EY (anionic) and MB (cationic) were incubated together in the same vial with one bead (Beada) as adsorbent for 6 h and the UV-Visible absorption spectra of the supernatant solution were recorded. The photographs of the dye absorbed beads were taken from the corresponding vials prepared at a concentration of 20 mg/L in case of each dye. The dye (CBB, BB and EY) adsorbed beads (dye@Beada) were analysed for their surface morphology by FEG-SEM. Specific Adsorption of Cationic Dyes by Beadb. The time dependent specific adsorption of cationic dyes, rhodamine 6G (R6G) and methylene blue (MB) was studied in a similar manner and quantified using UV-Visible spectroscopy. For these experiments, the anionic dye, EY was taken as control. The surface morphology of the dye adsorbed beads (dye@Beadb) were analysed by cryo SEM. The sample preparation is same as per that given for Beada studies in the previous section. The selective adsorption of MB from a mixture of {MB+EY} by Beadb was also studied using UV-Visible spectroscopy. The color change was also captured by digital imaging. The dye adsorption kinetics was studied in the same manner even in case of Beadb. pH Dependent Desorption of Dye and Reusability of Beads (Beada and Beadb ). The beads that adsorbed EY (EY@Beada) (100 mg/L) were incubated with 2 mL of 0.1 M NaOH to desorb the dye at different temperatures such as 4 ⁰C, 37 ⁰C and 50 ⁰C. Then the beads that were converted to Beadb at 37 ⁰C were separated out and were re-incubated in 0.1 M



acetic acid overnight to reconvert back to Beada for reusability {P.S.: Alternatively, it can be used as Beadb itself for the removal cationic species.} The time dependent desorption of EY was quantified by UV-Vis absorption spectroscopy. The same bead was reused successively for eight cycles of adsorption-desorption quantitatively and thus demonstrated the reusability of the beads. The cryo SEM was done at different stages of the adsorption-desorption cycles in order to assess the surface morphology of the beads upon recycling. Similarly, the beads possessing MB (MB@Beadb) (100 mg/L) were incubated in 0.1 M acetic acid in a tube rotator to desorb the dye at 4 ⁰C, 37 ⁰C and 50 ⁰C. Then the beads that were converted to Beada at 37 ⁰C were transferred to another vial having 0.1 M NaOH in order to convert it back to Beadb so that the next cycle can be carried out. {P.S.: Alternatively, the Beada can be used for the removal of anionic species.} The amount of desorbed dye was quantified by UVVis spectroscopy. The reusability of the beads was shown for 8 cycles of adsorptiondesorption of dyes. Demonstration of Water Purification taken through Adsorption-Desorption Cycles. Thirty-five to forty Beada were used in order to pack a column which was then loaded with 30 ml of 5 mg/L of EY. This was incubated for 1 hr and then collected dropwise in a beaker and checked for UV-Vis spectra to quantify the left over EY. The adsorbed dye was later released in 2 ml of 0.1 N NaOH. During the release process, the beads are converted to Beadb. To these Beadb, 30 ml of 5 mg/L of MB was added and now the cationic dye was absorbed by these beads (MB@Beadb) and the MB is released in acidic medium so as to regenerate Beada in order to continue for the next cycle. A mixture of dyes, i.e., {EY+MB} (10 mg/L, 100 mL) was also purified in two step process by first incubating it with Beada (20 in number) followed by another Beadb (20 in number) and the same can be executed in a reverse manner where the Beadb is used first followed by the use of Beada. The extent to which the dye was removed was quantified by UV-Visible spectrophotometer.


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Removal of Heavy Metal Cations (Pb2+, Cd2+ and Hg2+) from Contaminated Water. In order to quantify the % removal of toxic metal ions, i.e., Pb2+, Cd2+ and Hg2+, the ICP-AES analysis were performed. Ten mL of freshly prepared 100 ppm solutions of perchlorate salts of Pb2+, Cd2+ and Hg2+ were incubated with two of Beada and Beadb separately for 12 hrs in tube rotator at room temperature and the supernatant solutions were analysed for their metal content by ICP-AES. The competitive removal of these ions was studied using a mixture of {Pb2+ + Cd2+ + Hg2+} in a similar manner. The surface morphology of the metal ions adsorbed Beadb (Pb2+@Beadb, Cd2+@Beadb and Hg2+@Beadb) were analysed under cryo SEM. The maximum adsorption capacity was calculated in all the cases by incubating one of Beadb with 10 mL of varying concentration of Pb2+, Cd2+ and Hg2+ (2-100 ppm) for 24 hrs. The pH induced recyclability of Beadb was checked in case of Beadb@Cd2+ for 5 rounds of cycles by changing the pH of the Beadb to acidic and then back to basic after desorption. The surface morphology of the beads was checked by SEM at various stages of the cycles. Removal of Colored Anions from Water by Beada. Ten mL of freshly prepared 100 ppm solutions of each of KMnO4, HAuCl4 and K2PdCl4 was incubated with two of Beada and Beadb separately on a tube rotator for 12 hrs and then the beads were removed. The amount of anion removed from the solution was quantified using ICP-AES and the data were compared against appropriate controls. The changes in the color of the solution upon adsorption of anions were visible by the naked eye and the vials were photographed. The time dependent adsorption of the anions (200 ppm; 2 mL) by Beada was also carried out using UV-Visible spectrophotometer. In case of mixture of anions, {KMnO4+ HAuCl4}, time dependent UV-Visible absorption spectra were measured. The maximum adsorption capacity of KMnO4, HAuCl4 and K2PdCl4 by Beada was calculated by incubating one of Beada with 10 mL of varying concentration of KMnO4, HAuCl4 and K2PdCl4 (2-50 ppm) for 24 hrs.



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RESULTS AND DISCUSSION The BSA_Au beads (Beada and Beadb) were synthesized starting from BSA_Au NCs followed by the addition of glutaraldehyde and glycerol and then adding this mixture dropwise into the acidic medium (Beada) and then transferring the beads to the basic medium (Beadb) as given in Scheme 1.

pH=11-12

+ Glycerol + Glutaraldehyde

BSA

Soln. of HAuCl4

BSA_Au NCs

Beada

Beadb

Scheme 1: Schematic representation for the preparation of Beada and Beadb. Synthesis and Characterisation of BSA_Au NCs. The BSA_Au NCs were synthesized as per the details given in the supporting information (SI 01). The red fluorescent BSA_Au NCs were characterised by spectroscopy and microscopy and the corresponding data is given in Figure 1. The absorption spectra showed a peak at 280 nm that is characteristic of the protein and no SPR band was observed (Figure 1 a). The fluorescence spectrum shows the emission maximum at 650 nm when excited at 500 nm confirming the red fluorescence for BSA_Au NCs (Figure 1 b). An intense red fluorescence of Au NCs was seen under UV light in dark as given in the inset of figure 1 a. The BSA_Au NCs are of (2.95 ± 0.56) nm size as observed from TEM micrographs (Figure 1 c). The oxidation state of Au in BSA_Au NCs was analysed by XPS which shows peaks at 82.4 and 86.06 eV for Au0 (Figure 1 d). Also, peaks corresponding to Au+ were seen at 84.58 and 88.39 eV, but their peak areas were much smaller.


4

Absorbance

3 2

(a)

300 (b) BSA BSA_Au NC

200

0 200

(c)

Excitation Emission

100

1 400

600

 (nm)

0 400 500 600 700 800

800

2.94 ±0.56 nm

 (nm)

160

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity

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(d)

Au 4f5/2

Au 4f7/2

120 80 80

84

88

92

Binding energy(eV)

Figure 1: (a) UV-Vis absorption spectra of simple BSA (black) and BSA_Au NCs (red). Inset shows the photographs of vials under UV light. (b) Excitation (blue) and emission (green) spectra for BSA _Au NCs. (c) TEM micrograph for BSA_Au NCs (scale = 2 nm). (d) XPS spectra of BSA_Au NCs in the Au region. The de-convoluted peaks are seen.

Synthesis and Characterization of BSA_Au Beads. It is known in the literature that when the pH of the medium containing Au NPs/NCs is brought to acidic, these tend to aggregate.29,30 In the present case, an unprecedented method is demonstrated where the BSAAu NCs prepared at alkaline pH along with glutaraldehyde and glycerol results in bead like structure when the mixture is added drop-wise to 5% acetic acid solution. The aggregation of Au NCs is responsible for the formation of BSA_Au beads and this is supported by complete quenching of the red fluorescence once the beads are formed (Figure S1) as observed under UV light. The digital photographs of the beads in acidic (Beada) as well as in basic (Beadb) medium are shown in Figure 2 a, b. The beads are swollen in basic pH. The average diameter of the Beada is (3.4 ± 0.2) mm and that of Beadb is (5 ± 0.5) mm (Figure 2 c) supporting ~150% increase in the size on going from acidic to that of alkaline medium. The swelling of the beads is attributable to the change in the overall charge of the beads that arises from the deprotonation of the protein side chains. The average weight of the Beadb is increased to



~300% as compared to Beada (Figure 2 d). Overall, the density of Beadb is greater by ~110% as compared to Beada when filled with water. The amount of water content present in Beada and Beadb were quantified using TGA and the study showed ~75% and ~88 % respectively (Figure S2). Thus, the Beadb has greater content of water. However, the onset of loss of protein content is seen around 250 °C. (a)

(c)

0.4 0.2 0.0

(b)

Avg. wt. (mg)

0.6

Diameter (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Beada

Beadb

50 (d) 40 30 20 10 0

Beada

Beadb

Figure 2: Photographs of (a) Beada and (b) Beadb along with physical scale. Histogram plots of, (c) average diameter and (d) average weight of the beads. The error bar was derived based on measuring 20 different beads in each case.

The surface morphology of BSA_Au beads was analysed under SEM using cryo mode. A highly regular and porous structure was seen in case of Beada (Figure 3 a). The average size of the pore is (18 ± 3.2) µm. The morphology of the beads obtained from the acidic (Beada) and from the basic (Beadb) medium varies as the regular pores of Beada are converted to nonregular, porous honey comb like structure in Beadb. The pore size distribution plot also supports the non-regular porous nature of Beadb as can be noticed from Figure S3. However, the microstructure of beads is inter-convertible (Beada ↔ Beadb) and these are given in Figure 3.


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(a)

(b)

(c)

Figure 3: SEM micrographs of (a) Beada, (b) Beadb, and (c) Beada that was generated by immersing Beadb into acetic acid. The scale bar is 10 µm in all the cases.

The presence of Au in the lyophilised beads was confirmed by mapping the elements using SEM as well as TEM. The presence of Au in addition to C, N, O and S are seen and these are well distributed on the surface of the bead (Figure 4 a-c and Figure S4). The TEM micrograph (Figure 4 d and Figure S5) supports the aggregation and crosslinking of BSA_Au NCs. The amide bands of BSA at 1643 and 1513 cm-1 remains unaltered in case of BSA_Au NCs and BSA_Au beads (Figure 4 e). The oxidation state of Au in BSA_Au beads was determined by XPS which shows peaks at 83.23 and 87.70 eV that corresponds to Au (0) (Figure 4 f). However, peaks for Au+ were also seen.



(a)

(b)

Merged

(d)

(c)

(f)

(e)

%T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000 3000 2000 1000

 (cm-1)

Figure 4: (a) SEM micrograph of lyophilised Beada. (b) The lyophilised Beada given in (a) is imaged for all the elements present in it. The colour code is, Au (Red), C (Blue), N (Green), O (yellow), S (purple). (c) EDAX of Beada. (d) TEM micrograph of dispersed sample of Beada (Scale = 0.2 µm). (e) FT-IR spectra of BSA (black), BSA_Au NC (red) and Beada (blue). (f) XPS spectrum of Beada in Au region. The de-convoluted peaks are seen.

Selective Adsorption of Anionic Dyes by Beada.

The adsorption of organic dyes is

important in the development of water purification system. The porous BSA_Au beads (Beada) were checked for their dye adsorption efficacy and found that these are well suited for the adsorption of anionic dyes and not for cationic ones as can be understood from Figure 5 a. The time dependent adsorption of dyes is given in Figure 5 b, c and in the supporting information under Figure S6 which shows that the maximum adsorption occurs within 10 hrs. However, the time of adsorption can be reduced to 100 mins by taking 10 beads for the same study as shown in Figure S6. The negatively charged dyes, such as, EY, BB and CBB were adsorbed with high equilibrium adsorption capacity (qe) of (199 ± 7), (170 ± 6) and (227 ± 5) mg/g respectively by the Beada. However, the cationic dyes, such as, MB and R6G showed


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almost no adsorption by the same beads supporting that the Beada specifically adsorb only anionic dyes (Figure 5 d).

Figure 5. (a) Photographs of vials containing beads and dye: The upper panel correspond to vials prepared immediately after mixing the dye. The lower panel corresponds to the same after incubation for 10 hrs. Time dependent (0, 15, 30, 60, 90, 120, 180, 240, 300, 480, 600 min) UV-Vis spectra of supernatant solutions upon treatment with Beada: (b) EY and (c) MB. (d) Adsorption capacity (qe) of various dyes by Beada vs. time. The colour code for this is, EY (black), BB (red), CBB (blue), MB (olive green) and R6G (purple).

The time dependent adsorption data in case of all the three anionic dyes, viz; EY, CBB and BB fits well with a pseudo second order rate. The qcal values were derived from the corresponding fits and these are 175, 195, 313 mg/g for EY, BB and CBB respectively (Table S1). The maximum adsorption capacity was obtained from qe vs. Ce plot and the qmax(ex) = 245 mg/g for EY. The data fits better to the Langmuir adsorption isotherm and yields the maximum adsorption capacity (qmax(cal)) values of 278 mg/g for EY and this value agrees well with 245 mg/g that is observed experimentally (Figure S7).



The beads that adsorbed EY (EY@Beada) were lyophilised and characterised by UV-DRS and FT-IR which resulted in peaks for the protein as well as for the dye. The peak of the BSA at 280 nm is observed in all the cases, i.e., BSA, BSA_Au NCs, BSA_Au beads and EY@Beads. An additional peak at 530 nm is seen for EY in case of EY@Beada (Figure 6 a) due to the presence of the dye. Similarly, the peaks corresponding to EY and BSA are seen in FT-IR for EY@Beada (Figure 6 b). The competitive adsorption behaviour was checked using a mixture of cationic (MB) plus anionic (EY) dyes together. This experiment revealed selective adsorption of EY by the Beada, while the cationic dye, i.e., MB is remained in the solution as understood from Figure 6 c. The cryo SEM of EY@Beada, BB@Beada and CBB@Beada exhibits significant changes in the surface morphology where the pores of the beads were filled with the dye as noticed from Figure 6 d-f.

EY peak

0.1

(d)

400

600

 (nm)

%T

(b)

0.2

0.0 200

Absorbance

0.3

0.3 (a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800 4000 3000 2000 1000

 (cm-1)

(e)

(c)

0.2 0.1 0.0 400 500 600 700 800

 (nm)

(f)

Figure 6: (a) UV-Vis DRS spectra and (b) FT-IR spectra of EY@Beada and its controls. The colour code is: BSA_Au bead (Black), EY@Beada (Red) and EY (Blue). (c) UV-Visible absorption spectra for {EY+MB} before (red) and after (blue) incubation by Beada. Cryo


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SEM micrographs: (d) EY@Beada (Scale bar = 10 µm), (e) BB@Beada (Scale bar = 1 µm) and (f) CBB@Beada (Scale bar = 1 µm).

Selective Adsorption of Cationic Dyes by Beadb. The beads obtained from the basic medium (Beadb) are expected to have the nature of the charge that is opposite to that of the Beada and hence these beads are expected to adsorb cationic dyes selectively. It is indeed observed that the cationic dyes, viz, MB and R6G were selectively adsorbed by Beadb, while no anionic dye (EY) being absorbed. All this can be noticed from the time dependent adsorption data of the respective cationic dyes in case of MB and R6G, while no change is noticed in case of the anionic dye, EY (Figure 7 a, b, S8). Even in this case, the time required for the adsorption of MB can be reduced to 1 hr when 10 beads are taken in the experiment (Figure S8). Thus, the adsorption time is reduced by an order of magnitude when the number of beads used in the study was increased by a factor of 10. The time dependent UV-Visible absorption studies show high and selective adsorption capacity in case of MB and R6G, i.e., (239 ± 1), (183 ± 0.5) mg/g respectively (Figure 7 c). Even in this case, the adsorption data for MB as well as for R6G fits well with the pseudo second order model and data is given in Table S2. The photographs of vials before and after the dye adsorption are given in the inset of Figure 7 c. The maximum adsorption capacity of Beadb was experimentally obtained and the corresponding qmax(ex) = 313 mg/g in case of MB. The data from this was fitted to Langmuir isotherm which yields a value of 345 mg/g for qmax(cal) and thus agrees well with that of the qmax(ex) (Figure S9). The cryo SEM of MB@Beadb and R6G@Beadb shows distinct changes in the surface morphology where the pores are now filled with fibrillar structures as can be seen from Figure 7 d, e.

The selective adsorption of the cationic dye, MB has been



demonstrated when a mixture of {EY+MB} was taken for the experiment (Figure 7 f). In this, the anionic EY was left out in the solution. 1.2

1.0 0.5

(d)

600

700

 (nm)

qe (mg/g)

1.5

0.0 500

250 (c) 200 0.8 150 100 0.4 50 0 0.0 800 400 450 500 550 600 0 200

(b)

 (nm)

(e)

0.3

Absorbance

(a)

Absorbance

2.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R6G

MB

400

t(mins)

EY

600

(f)

0.2 0.1 0.0 400 500 600 700 800

 (nm)

Figure 7: Time dependent (0, 15, 30, 60, 90, 120, 180, 240, 300, 480, 600 min) UV-Vis spectra of supernatant solutions upon treatment with Beadb:

(a) MB and (b) EY. (c)

Adsorption capacity (qe) of various dyes by Beadb vs. time. The colour code is: MB (black), R6G (red) and EY (blue). Inset of (c) shows photographs of vials before (upper panel) and after (lower panel) incubation of respective dyes by Beadb. Cryo SEM image of dye@Beadb: (d) MB and (e) R6G. Scale bar is 1 µm in both the cases. (f) UV-Visible spectra for {EY+MB} before (Red) and after (Blue) incubation by Beadb.

pH Dependent Adsorption - Desorption of Dye & Recyclability and Convertibility of Beada and Beadb. In order to utilise the Beada efficiently, it is important to recycle and reuse it. To reuse it, the adsorbed dye should be desorbed. Since the adsorption of dyes is considered to be dependent on the nature of the charge, the reversal of the charge on the Beada should desorb the dye from the beads. The adsorbed dye was desorbed in solution


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


within 30 mins at 37 ⁰C when the EY@Beada were incubated in 0.1 N NaOH as quantified by the time dependent UV-visible spectroscopy (Figure 8 a). The desorption was also carried out at 4 ⁰C and 50 ⁰C and it is found that maximum desorption occurs at 37 °C only (Figure S10). The desorption of EY automatically converts Beada  Beadb and therefore the Beadb at this stage was converted back to Beada by treating with acid. Thus, the converted Beada was demonstrated for its reuse for 8 cycles and observed that the efficiency of adsorption and desorption of the Beada was well retained (Figure 8 b). The Beada which desorbed EY was analysed by SEM and it is seen that the surface morphology of Beada is retained even after the desorption of the dye (Figure 8 c). The same bead was used again for adsorption of EY and it showed similar surface morphological SEM features. Similarly, the adsorbed dye (MB) on the Beadb was desorbed within 20 minutes in acidic medium (Figure 8 d). The recyclability was checked for 8 rounds of adsorption-desorption cycles and it is observed that the adsorption capacity of Beadb is well retained (Figure 8 e) and thus the functional efficiency of these beads is not compromised to any significant extent over all the eight cycles demonstrated. The SEM micrograph measured after the desorption of dyes for 5 cycles also shows that the surface morphology of Beadb is retained and no dye fibres can be seen (Figure 8 f). The Beadb re-adsorbs the dye and the corresponding dye@Beadb showed similar fibrillar structures trapped inside the pores. Thus, the Beada and Beadb are inter convertible, recyclable and its adsorption and desorption processes are driven by the change in the pH of the medium.



qabs

1.0 100 0.8 0.6 0 0.4 (a) -100 0.2 0.0 400 450 500 550 600

 (nm)

qabs

1.6 1.2

(d)

(g)

(h)

1 2 3 4 5 6 7 8 Number of cycles

200 (f) 100 0

0.8 (e)

qdes

0.4 0.0 500

(c)

qdes

Absorbance

200 (b)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

600

 (nm)

700

-100 -200

1 2 3 4 5 6 7 8

Number of cycles

Figure 8: (a) Time dependent UV-Vis spectra for pH induced desorption of EY from EY@Beada (0 to 120 mins). Inset shows the photographs of EY@Beada on the left and vial containing desorbed EY solution on the right. (b) Recyclability of Beada for 8 rounds of adsorption-desorption cycles of EY. Cryo SEM image: (c) Beada after desorption of EY and (d) Beada upon re-adsorption of EY. The scale bar is 10 µm in both the cases. (e) Time dependent UV-Vis spectra for pH induced desorption of MB from MB@Beadb (0 to 60 mins).

Inset shows the photograph of MB@Beadb on the left and the vial containing

desorbed MB solution on the right. (f) Recyclability of Beadb for 8 rounds of adsorptiondesorption cycles of MB. Cryo SEM image: (g) Beadb after desorption of MB and (h) Beadb upon re-adsorption of MB. The scale bar is 1 µm in both the cases.

Demonstration of Water Purification from Organic Dyes through AdsorptionDesorption Cycles. In order to demonstrate water purification by the beads, an experimental set up was designed as shown in Figure 9 a. In the first step, the Beada was employed to adsorb the anionic dye, EY from water. The dye adsorbed by the Beada was desorbed in NaOH and thereby Beada is converted to Beadb which was again re-employed to purify the


Page 19 of 30

water from the cationic dye MB as shown in Figure 9 b-g. The UV-Visible spectra were measured in order to quantify the dye adsorbed and found that the water was purified from the dye in each case (Figure S11). Thus, Beada  Beadb and vice versa, i.e., Beadb Beada were achieved by changing the pH of medium, as a result both the anionic and cationic dyes are removed by the same set of starting beads (whether Beada or Beadb) in successive cycles. The purification of water from organic dyes was scaled up to 100 mL in order to demonstrate laboratory scale purification of water. In this case, the water containing MB and EY as dye contaminant (Figure 9 h) was incubated with Beadb in the first stage to adsorb MB from the mixture (Figure 9 i) and then incubated with Beada such that the EY is adsorbed (Figure 9 j) and thereby the water is removed from these contaminants and the extent of removal was quantified by UV- visible spectroscopy (Figure 9 k). The same experiment can be carried out by starting from Beada, since Beada and Beadb are inter-convertible.

(a)

(b)

(c) EY

+ Beadb

EY@Beada

(f)

+ Beada

+ NaOH  EY desorbed  Beadb

+ CH3COOH  MB desorbed  Beada

0.8

(e) Incubate

MB@Beadb

{EY + MB}

Incubate

Water purified of EY

(g)

(i)

(h)

(d)

MB

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Water purified of MB

0.6 0.4

inital after incubation with Beadb after incbubation with Beada

(j)

(k)

0.2 0.0 400 500 600 700 800

 (nm)

Figure 9: (a) Experimental setup for water purification. (b-g) Various steps of purification of contaminated water from organic anionic (EY) and cationic (MB) dyes by Beada and Beadb respectively as per the labelling given in the figure. Photograph of 100 mL beaker possessing (h) a mixture of {EY+MB}, (i) after incubation with Beadb, (j) after incubation with Beada.



(k) UV-Vis absorption spectra of the dye-mixture before (black) and after incubation with Beadb (red), followed by further incubation with Beada (blue).

Removal of Toxic Metal Cations (Pb2+, Cd2+ and Hg2+) from the Contaminated Water. The BSA_Au beads (Beada and Beadb) were employed for the removal of toxic heavy metal ions from water. The Beadb removes ~100% in case of Pb2+ and Cd2+, and (75±1.5) % in case of Hg2+ (Figure 10 a) owing to the presence of opposite-charge on the Beadb. The Beada does not remove Pb2+ and Cd2+ from water due to the charge on Beada being similar in nature. However, this removes Hg2+ to (52±2.5)% owing to its high affinity towards sulphur containing groups present in BSA.31 Even from the mixture, the Beadb removes (93±1.6), (70±2.6) and (58.6±2.6)% in case of Pb2+, Cd2+ and Hg2+ ions (Figure 10 b) respectively, while the Beada removes only Hg2+ to (50±3.9)% from this mixture and not the other two ions. This reaffirms the charge nature of the bead and its selective removal of ions having the

100 (a) 80 60 40 20 0

From mixture

(e)

Pb

2+

(f)

Cd

2+

qdes

% Removal

Pb2+ Cd2+ Hg2+ (d)

100 (b) 80 60 40 20 0

qads

opposite charge from water.

% Removal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

Hg

2+

(g)


300 200 100 0 -100 -200 -300

(c)

1 2 3 4 5 Number of cycles (h)

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Figure 10: (a) Percentage removal of Pb2+, Cd2+ and Hg2+ by Beada (Red bar) and Beadb (Blue bar) from the solutions containing these ions. (b) Competitive removal of Pb2+, Cd2+ and Hg2+ from a mixture when all these three cations are taken together. In both (a) and (b) the black bar shows the control (no addition of either Beada or Beadb). (c) Recyclability of Beadb for 5 rounds of adsorption-desorption cycle of Cd2+. Cryo SEM micrograph (in backscatter mode): (d) Pb2+@Beadb (Scale bar = 1 µm), (e) Hg2+@Beadb (Scale bar = 1 µm) and (f) Cd2+@Beadb (Scale bar = 10 µm) (g) Beadb obtained after desorption of Cd2+ ions (Scale bar = 1 µm) and (h) Beadb after re-adsorption of Cd2+ (Scale bar = 10 µm).

The recyclability of Beadb was analysed in case of Cd2+@Beadb in which Cd2+ was desorbed in acidic medium and again the same bead was used for another 5 cycles where the efficiency of Beadb is well retained (Figure 10 c). The adsorbed metal ions, viz., Pb2+@Beadb, Cd2+@Beadb and Hg2+@Beadb can be noticed from the features observed on their surface in the corresponding SEM images given in Figure 10 d-f. The SEM images are taken in backscatter mode where higher contrast of metal ions can be seen. The maximum adsorption capacity of Cd2+, Pb2+ and Hg2+ by Beadb are 327, 270, 208 mg/g respectively as derived based on ICPAES analysis (Figure S12). The SEM micrographs of Cd2+ desorbed from Cd2+@ Beadb shows that the initial structure of Beadb is retained and no contrast of metal ion is seen in the backscatter mode supporting the absence of Cd2+ deposition on the surface (Figure 10 g). The same bead was again used to re-adsorbing the Cd2+ which again showed the deposition of metal ion on the surface (Figure 10 h). Removal of Colored Anions from Water by Beada. The colored anion contaminants such as KMnO4, K2PdCl4 and HAuCl4 were adsorbed on Beada and the photograph of the corresponding vials is given in Figure 11 a. The time dependent UV-Visible spectra show



decrease in the specific absorption band of the respective anion (Figure 11 b, c, d). The surface morphology of the anion adsorbed Beada (viz., MnO4-@Beada, AuCl4-@Beada, PdCl42-@Beada) was studied by cryo SEM as given under Figure 11 e, f and g, where a greater contrast results from the adsorbed anions on the surface of Beada. The maximum adsorption capacity of KMnO4, HAuCl4 and K2PdCl4 by Beada is 44, 88 and 59 mg/g as estimated based on the ICP-AES data given in Figure S13. The adsorption of anions by Beada can also be seen when a mixture of anions, i.e., {KMnO4 + HAuCl4} was taken (Figure 11 h). The anions are selectively adsorbed by Beada to the extent of 85, 79 and 56% in case of KMnO4, HAuCl4 and K2PdCl4 respectively also obtained from ICP-AES data (Figure 11 i). However, Beadb only shows minimal adsorption in all the three cases owing to its likecharge. 0.20

0.04

(b) Absorbance

Absorbance

(a)

0.15 0.10 0.05 0.00 450 500 550 600 650

(c)

0.03 0.02 0.01

 (nm)

1.2

400

500

 (nm)

600

(d) (e)

0.8

(f)

0.4 0.0

(g)

250

300

 (nm)

350

0.8

(h)

100 80 60 40 20 0

(i)

% Removal of anions

-0.4 200

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

0.6 0.4 0.2

0.0 200 300 400 500 600 700

 (nm)


MnO-4 AuCl-4 PdCl24

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11: (a) Photographs of vials before (upper panel) and after (lower panel) incubation of respective anion solutions with Beada. Time dependent (0 to 300 mins) UV-Vis absorption spectra upon incubation with Beada: (b) KMnO4, (c) K2PdCl4 and (d) HAuCl4. Cryo SEM images (in backscatter mode): (e) KMnO4@Beada (scale bar = 1 µm), (f) K2PdCl4@Beada (scale bar = 10 µm) and (g) HAuCl4@Beada (scale bar = 10 µm). (h) Time dependent (0 to 180 min) UV-Visible spectra of mixture of anions upon incubating with Beada.

(i)

Percentage removal of anionic species of KMnO4, HAuCl4 and K2PdCl4 by Beada (Red bar) and Beadb (Blue bar) as obtained from ICP-AES data. Black bar shows the control where there is no addition of either Beada or Beadb.

CONCLUSIONS AND COMPARISONS In this paper, a new and unprecedented strategy has been developed in order to synthesize BSA beads from BSA_Au NCs. These beads showed interesting inherent properties such as high porosity, micro structural reversibility and pH responsiveness. Owing to their highly porous nature, these beads (Beada and Beadb) were employed for the adsorption of organic and inorganic contaminants of both cationic and anionic ones from water. It is interestingly found that Beada shows selective and very high adsorption capacity only for anionic dyes and simple and complex inorganic anionsand those of Beadb shows selective adsorption for only cationic dyes and heavy metal cations. The adsorption kinetics in case of the dye adsorption follows pseudo second order and fits better with Langmuir isotherm in case of both types of beads with all the dyes studied. The adsorbed dye on the Beada has been desorbed in alkaline pH and therefore are reused for 8 rounds of cycles without losing its efficiency. Similarly,



Page 24 of 30

the adsorbed dyes in the Beadb have been desorbed in acid medium and were shown for their recyclability for several cycles. The highlights of this can be understood from Scheme 2. Beada-EY

AD

AD@Beada EY@Beada

Beada

Beadb-MB

AD released

CD released

H+ CD Beadb CD@Beadb

OHBeadb

IC

MB@Beadb

OH-

IC@Beadb AuCl4-@Bead a

IA

H+

IA@Beada Beadb

Cd2+@Beadb

- Cd2+

IC released

H+

OHAD = Anionic dye CD = Cationic dye IA = Inorganic anion IC = Inorganic cation

Scheme 2: Flowchart showing the pH responsive behaviour of Beada in de-contaminating water from organic and inorganic species of cationic as well as anionic types. Similar scheme can be constructed starting from Beadb but by using CD in place of AD, and IA in place of IC, while inter-converting H+ and OH- of the medium appropriately. The surface morphology features of the adsorbed as well as the recycled beads were further demonstrated by cryo SEM studies. Therefore, the biologically benign BSA_Au beads are demonstrated to be smart, pH responsive for efficient removal of organic and inorganic species of both cationic and anionic contaminants with high adsorption capacity and high selectivity and therefore can be very well utilised for routine water purification. ASSOCIATED CONTENT Supporting information available: The supporting information includes procedure for synthesis of BSA_Au NCs and the photographs of the lyophilized powder, TGA of Beada and Beadb, SEM image of lyophilised Beada, Elemental mapping of Beada in by STEM, UVVisible spectra of supernatant solution of different dyes in presence of Beadsa, Parameters


Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


such as R2, K1, K2 and qcal for pseudo first order fit and second order fit for EY, BB and CBB, absorption spectra of R6G by Beadb, Parameters for pseudo first order fit and second order fit for MB and R6G, Langmuir and Freundlich isotherm fitting for MB, UV-Vis spectra of EY and MB before and after passing through column set-up, maximum adsorption capacity of Pb2+, Cd2+ and Hg2+ by Beadb and maximum adsorption capacity of KMnO4, K2PdCl4 and HAuCl4 by Beada. AUTHOR INFORMATION Corresponding Author: To whom correspondence should be addressed. Phone: 91 22 2576 7162. Fax: 91 22 2572 3480. Email: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS CPR acknowledges financial support from the DST/SERB {EMR/2014/000985} and for J. C. Bose National Fellowship {SB/S2/JCB-066/2015}, and IIT Bombay for Institute Chair Professorship. AU acknowledges UGC for the award of Senior Research Fellowship {Ref. No. 21/12/2014(II) EU-V; Serial No. 2121410051}. We acknowledge the services provided by the central facilities of IIT Bombay, viz., cryo SEM, XPS, TEM, ESEM and ICP-AES.

DEDICATION We dedicate this paper to Professor C.N.R. Rao, F.R.S., on his 85th birthday.

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Graphical Abstract


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Porous, pH Responsive and Reusable Hydrogel Beads of Bovine

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