Interaction of Ionic Liquid with Silver Nanoparticles: Potential

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11088−11100

Interaction of Ionic Liquid with Silver Nanoparticles: Potential Application in Induced Structural Changes of Globular Proteins Manoj Kumar Banjare,†,‡ Kamalakanta Behera,§ Ramesh Kumar Banjare,†,∥ Reshma Sahu,† Srishti Sharma,† Siddharth Pandey,⊥ Manmohan L. Satnami,† and Kallol K. Ghosh*,† †

School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh 492 010, India Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh 495 009, India § Centre for Interdisciplinary Research in Basic Sciences, JMI, Jamia Nagar, New Delhi 110 025, India ∥ School of Biological and Chemical Science, MATS University, Raipur, Chhattisgarh 492001, India ⊥ Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

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‡

S Supporting Information *

ABSTRACT: Silver nanoparticles (AgNPs) show immense application potential in many fields including biomedical sciences owing to their advanced antimicrobial property (antibacterial, antifungal and anti-inflammatory). Ionic liquids (ILs) known as designer solvents with a melting point below 100 °C are a new class of compounds with exclusive properties and great chemical variety. Since the past decade, ILs are vastly used in interdisciplinary research including the synthesis and stabilization of metal nanoparticles. In the present work, we have studied the interaction between IL 1-butyl-3-methylimidazolium octylsulfate [Bmim][OS] and AgNPs using a simple and sensitive UV−visible spectroscopic method via the response obtained from the surface plasmon resonance (SPR) band of AgNPs. The analysis of the SPR band at the 400−425 nm range and the Fourier transform infrared (FT-IR) spectral results show a transfer of N−H stretching frequency from 3367 to 3228 cm−1, which clearly suggests the formation of nanoparticles. FT-IR spectroscopy is systematically applied to explore information about the intermolecular interactions taking place within AgNPs-[Bmim][OS] micellar solutions. The critical micelle concentration (CMC) of [Bmim][OS] is determined using conductivity and UV−visible spectroscopy, and the size of micellar aggregates is obtained using dynamic light scattering (DLS) technique. Further, the above system is also utilized to consider the structural modify of human serum albumin (HSA) and bovine serum albumin (BSA). UV−vis, fluorescence, FT-IR, 1H nuclear magnetic resonance and DLS spectroscopic investigations reveal some interesting outcomes. It is observed that modification in the structures of both the globular proteins HSA and BSA takes place within the system, thus indicating significant IL−protein binding. Further, it is noticed that HSA shows more binding affinity toward [Bmim][OS] than compared to BSA. KEYWORDS: Silver nanoparticles, Ionic liquid, Serum albumins, Spectroscopy

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INTRODUCTION The vast application of metallic nanoparticles (NPs) has brought curiosity among researchers in many research fields, viz., plasmonics, photonics, biological and chemical sensors, cell electrodes, optical devices and antimicrobial activities because of their characteristic optical, chemical and physical properties.1−3 More importantly, silver nanoparticles (AgNPs) have shown widespread application potential based upon properties that have defined their shape, size, configuration and crystal orientations.4,5 AgNPs possess a high surface-to-volume ratio with exclusive optical, electrical and thermal properties for novel applications, such as pharmaceuticals, biological sciences, mechanics, packaging, food science, electronic device and information technology, etc.6,7 These NPs are very useful in the field of biomedical science owing to their superb antimicrobial properties.8 AgNPs can be effectively used for reducing bacterial adhesion to dental implant surfaces and © 2019 American Chemical Society

preventing biofilm formation. Apart from their antibacterial activities, AgNPs also possess antifungal and anti-inflammatory properties and are widely used as sore curative agents leading to their application in industrial antimicrobial covering.9−12 AgNPs have been reported by many researchers as the most efficient antimicrobial agent of choice against the development of antibiotics-resistant strains of microorganisms.13−15 In recent times, synergetic antibiotics of penicillin G, vancomycin, ampicillin, erythromycin, amoxicillin, clindamycin, chloramphenicol and kanamycin containing AgNPs have been reported.16−18 The antimicrobial activities of AgNPs are seen to be dependent upon the concentration of NPs in the medium and NPs shapes and sizes.19,20 Hence, it is of great importance Received: December 16, 2018 Revised: April 29, 2019 Published: May 29, 2019 11088

DOI: 10.1021/acssuschemeng.8b06598 ACS Sustainable Chem. Eng. 2019, 7, 11088−11100

Research Article

ACS Sustainable Chemistry & Engineering

surface plasmon resonance (SPR) spectrum via UV−vis spectroscopy and observed a red shift. However, the interactions between IL, proteins (human serum albumin and bovine serum albumin) and AgNPs have been barely reported. Herein, we have reported for the first time a systematic investigation on the interactions between AgNPs with IL [Bmim][OS] and further utilized this ILAgNPs system to study the structural changes of two globular proteins (HSA and BSA). In the present work, the synthesis and characterization of AgNPs are carried out using different techniques and the response from surface plasmon resonance (SPR) band has been utilized to investigate the CMC of the short-chain IL 1-butyl-3-methylimidazolium octylsulfate [Bmim][OS] using UV−vis spectroscopy. The sizes of AgNPs and AgNPs-[Bmim][OS] systems have been studied using DLS and FT-IR spectroscopic investigation have been performed to study the interactions taking place in these systems. Diethylene triamine (DETA) reduces silver ions (Ag+) during in situ formation between 400 and 430 nm, which is studied using UV−vis spectroscopy at different temperatures. Structural modification of two globular proteins, human serum albumin (HSA) and bovine serum albumin (BSA), via strong interactions with the IL [Bmim][OS] has been explored by using diverse spectroscopic methods. Our results gained from the multispectroscopic investigations give precious information on IL−protein ([Bmim][OS]-BSA/ HSA) binding. Molecular structures of ionic liquid 1-butyl-3methylimidazolium octylsulfate, diethylene triamine, human serum albumin and bovine serum albumin are presented in Scheme S1.

to optimize numerous factors that control the shapes and sizes of NPs. Therefore, to improve the applicability of AgNPs, it is essential evolve cost-effective, eco-friendly, simple and stable methods for fabrication of AgNPs.21 Ionic liquids (ILs) are presently a growing area of major research focus worldwide owing to their unique properties and widespread application potential.22−25 ILs are a novel class of compounds, which are composed of asymmetric organic cations and symmetric organic or inorganic anions.23−25 ILs show a tendency to self-aggregate in solvents and also associate with other additives, such as drugs, cyclodextrins, surfactants and proteins, to form a variety of molecular assemblies.26−30 These unique compounds have achieved much interest in academia as well as in industrial research communities all over the world due to their exclusive properties viz., nonflammability, high stability, very low vapor pressure and ionic conductivity which can be tuned to the structure of ions.31−34 ILs are widely applied in many research areas, i.e., separation, catalysis, electrochemistry, extraction, synthesis and biocatalytic processes, among many others.35−40 Currently, interaction between metallic nanoparticles (MNPs) and IL has gathered much attention in the field of nanotechnology.41−43 In recent years, scientists are trying to synthesize metal nanoparticles in green/environmental friendly and designer solvent IL, which is more important from environmental point of view.44 Nowadays, long chain imidazolium based ILs provide a great opportunity to replace the conventional surfactants used in the study on nanoparticles. 45−47 These ILs are green or toxicologically eco-friendly as compared to common surfactants.48 ILs give directional, entropic drivers and a stable ionic channel that contribute to electrostatic forces on the surface of NPs which can be utilized as stabilizers for NPs when synthesized in aqueous solution.49 A thorough study of NPs dispersion in ILs can therefore offer an enhanced basic kind on the novel applications of ILs in NPs synthesis, in chemical reactions involving NPs and in hybrid materials that include ILs and NPs.49 AgNPs show an absorption band which is identified as surface plasmon resonance (SPR), usually sited in the visible region and establishes the visual properties of the NPs. Plasmon energy depends on size and shape for a nanostructure.50−52 The SPR band of AgNPs is situated in the visible area and the band is highly dependent on NPs composition, size, shape, crystallinity as well as interparticle spacing.53−56 Transfer on the SPR band as well as color transformation have been used to resolve critical micelle concentration (CMC) of amphiphilic molecules.57 Salem et al.58 have reported the CMC of bis(2-ethylhexyl) sulfosuccinate sodium salt and sodium dodecyl sulfate from the SPR band sensitivity w.r.t. surfactant concentration. Diethylene triamine is used for reduction of Ag+ in AgNPs during situ formation. Mukherjee et al.59 studied the interaction of silver nanorods (SNRs) with bovine serum albumin (BSA) by circular dichroism (CD), dynamic light scattering (DLS), ζ-potential and FT-IR methods. DLS as well as ζ-potential confirm the binding of BSA toward the SNRs. The influence of SNRs on the hydrolysis of BSA was well explored by assaying the esterase activity. Ali and group60 synthesized polyvinylthiol coated AgNPs (Ag-PVT) and studied their interaction with human serum albumin (HSA) characterized by UV−visible, fluorescence and CD. It was shown that HSA protein partly unfolded in Ag-PVT NPs. Duyne et al.61 synthesized AgNPs and functionalized them with alkanethiol. They studied the

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EXPERIMENTAL SECTION

Materials. Ionic liquid 1-butyl-3-methylimidazolium octylsulfate (≥95.0%(HPLC)), silver nitrate (ACS reagent, ≥99.0%), diethylene triamine (Reagent Plus, 99%), human serum albumin (≥98.0% (SDSPAGE)), bovine serum albumin (lyophilized powder, crystallized, ≥98.0% (GE)), sodium borohydride (≥96% (gas-volumetric)), trisodium citrate (≥99%, FG) and potassium bromide (FT-IR grade, ≥99% trace metals basis) were purchased from Sigma-Aldrich Pvt. Ltd., Bangalore, India. Deuterium oxides (99.9% for nuclear magnetic resonance (NMR) spectroscopy grade) were purchased from Merck KGaA, Darmstadt, Germany. All the solutions have been prepared using Millipore water. Methodology. Initially, 100 μL of DETA was added to a given amount of IL. Thirteen samples of different [IL] were used in this work: ([Bmim][OS] = 3.3, 8.3, 11.6, 13.3, 16.6, 18.3, 20.0, 21.6, 23.3, 25.0, 28.3, 30.0 and 33.3 mM). The [Bmim][OS] concentrations cover the range both below and above the CMC. The solutions were thoroughly mixed. A 10 mL solution of AgNO3 (1 mM) was slowly added into each sample of the above solutions of IL and DETA. These solutions were shaken and kept in the dark at 293, 297 and 301 K. IL is used as a capping agent that is added to coat the nanoparticles and to avoid this agglomeration. DETA is used as a stabilizing agent. Synthesis and Characterization of AgNPs. In a simple method, 50 mL of silver nitrate solution (1 mM) and 1.0 mL of trisodium citrate (10 mM) were mixed in a 100 mL volumetric flask and vigorously stirred. After 15 min, 50 μL of sodium borohydrate (100 mM) solution was speedily inserted into this mixture solution and the resulting color changed to pale yellow and then to turquoise. During this point, the NPs solution was cooled at room temperature and centrifuged at 5000 rpm for 10 min. The supernatant was discarded and the pellet was air-dried in the incubator. The characterization of AgNPs was done using UV−vis, DLS and FT-IR spectroscopy. HSA/BSA with AgNP-[Bmim][OS] Suspension. AgNP[Bmim][OS] solution was mixed in distilled water and then sonicated to gain a uniform mixture. 0.01 M concentration was used for both 11089


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Figure 1. Characterization of the synthesized AgNPs [A] UV−vis spectrum of AgNPs, [B] UV−vis spectra of AgNPs+DETA, [C] AgNPs size distribution obtained from DLS at 298 K and [D] FT-IR spectrum of AgNPs prepared using sodium borohydrate and trisodium citrate. at 400 MHz). Chemical shift values for BSA/HSA-[Bmim][OS]AgNPs complexes were determined in deuterium oxide (D2O) solutions.

BSA and HSA. A mixture of IL-AgNPs and HSA/BSA (1:1) was mixed thoroughly and kept for 60 min. The IL−AgNP−protein mixture was further used for various spectroscopic investigations to analyze interactions. Conductivity. Conductivity studies were done using a Systronics Type (Model 306) digital meter, and cell constant was standardized with two different concentrations of potassium chloride (0.01 and 0.1 M) solutions. The standard [Bmim][OS] solution was gradually added in AgNPs solution and conductance was observed once thoroughly mixed at 298 K. UV−visible Spectroscopy. AgNPs absorption spectra were recorded using a UV−visible spectroscopy instrument (Varian Carry-60, Agilent Technology). UV spectra were recorded after 5 min from the addition of AgNPs solution in the wavelength range 200−800 nm. UV−vis absorption spectra of the protein solutions, i.e., HSA-AgNPs-[Bmim][OS]-DETA and BSA-AgNPs-[Bmim][OS]DETA systems, were studied in the wavelength range 200−400 nm at 298 K. Dynamic Light Scattering. The DLS technique was utilized to study the particle size of AgNPs and mixture for IL with AgNPs, DETA, BSA and HSA. DLS experiments were carried out using Malvern Zeta sizer Nano (Malvern Instruments Ltd., UK), and the intensity of scattered light was maintained at 90° and temperature at 298 K. Fourier Transform Infrared. FT-IR spectra of AgNPs, IL [Bmim][OS], DETA, HSA, BSA and AgNPs-DETA, AgNPs-DETA[Bmim][OS], BSA-AgNPs-[Bmim][OS]-DETA and HSA-AgNPs[Bmim][OS]-DETA systems were studied using a Nicolet iS10 (Thermo Fisher Scientific Instrument, Nadison, USA) FT-IR spectrophotometer. The FT-IR instrument was calibrated with 32 scans at 4 cm−1 resolution and spectral range between 4000 and 450 cm−1. Fluorescence. The fluorescence spectroscopic studies were carried out using the Carry eclipsed fluorescence spectrophotometer (Agilent Technology). Fluorescence excitation was done at 278 nm with emission and excitation slits set at 2.5, 5 nm, and emission spectra were taken between 280 and 600 nm. The concentration of protein in the HSA/BSA/AgNPs/DETA system was held constant and the concentration for IL [Bmim][OS] was changed in the solution. Nuclear Magnetic Resonance. The 1H NMR spectra of our systems were studied utilizing a Bruker NMR spectrometer (working

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RESULTS AND DISCUSSION Morphological Characterizations of AgNPs. The mechanism for the synthesis of AgNPs using trisodium citrate is shown as follows: 4Ag + + C6H5O7 Na3 + 2H 2O → 4Ag 0 + C6H5O7 H3 + 3Na + + H+ + O2 ↑

The synthesized AgNPs are characterized by utilizing the different spectroscopic techniques, i.e., UV−visible, DLS as well as FT-IR spectral analysis (Figure 1). The UV−vis absorption studies can be assumed as the key method for structural classification of AgNPs. The UV−vis spectra of nanoparticles confirm extremely symmetric single-band absorption with peak maxima (surface plasmon resonance (SPR)) at wavelength 425 nm.62 From the UV−vis spectrum of AgNPs (Figure 1A), a broad absorption peak at 420 nm indicates the formation of AgNPs whose spectrum shows no other peak, which confirms that the synthesized yields are AgNPs only. Addition of DETA (10 μL) to AgNPs solution at different time intervals results in shifting of the absorption peak of AgNPs (425 to 400 nm), and the observed blue shifts are represented in Figure 1B. The average particle size obtained from DLS experiments is 75.91 nm and PDI 0.234 (Figure 1C). The exterior brown color (Figure 1, inset) is the sign of the formation of AgNPs in the medium.63−65 Further, FT-IR spectroscopic investigation was done to identify the feasibility of biomolecules responsible for allowing trisodium citrate and NaBH4 for the AgNPs synthesis by top down methods. Three evident IR bands are seen at 3453.65, 1637.41 and 1384 cm−1 (Figure 1D). An important and very sharp peak is observed at 1384 cm−1, which is due to the nitrate ions. The broad band at 3453.65 cm−1 is owing to N H and OH stretching present in proteins. The middle strong 11090


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Figure 2. [A] UV−visible absorption spectra of AgNPs in the presence of IL [Bmim][OS] and [B] absorption band intensity against concentration of [Bmim][OS] (mM), at 293, 297, and 301 K temperature, respectively.

Scheme 1. Synthesis of AgNPs and Their Interaction with DETA and IL [Bmim][OS]

band at 1637 cm−1 occurs from the CO stretching mode in amide I group generally present in globular proteins, representing the presence of proteins as a capping agent for AgNPs that increases the stability of the NPs synthesized. More importantly, the broad and sharp IR peak at 386 cm−1 corresponds to silver metal. UV−vis Spectroscopy. From the UV−vis spectral response of AgNPs, a peak maximum was observed at λmax = 420 nm.66 The change in spectral response of synthesized AgNPs in the presence of IL [Bmim][OS] was used to prove the development of the IL−AgNp complex. Since [Bmim][OS] shows no absorption throughout the wavelength range 200−600 nm, its contribution to the UV−vis spectra can be ignored. It is noticed that Ag+ ion shows increased intensity at 420 nm due to the reduction event between IL [Bmim][OS] and AgNPs as shown in Figure 2. Gudikandula et al.66 synthesized colloidal AgNPs, which was observed through UV−visible spectroscopic response. AgNPs show the UV−vis peak around 420 nm, which is parallel on the surface plasmon absorbance to AgNPs. AgNPs are classified by (i) size and (ii) shape, which were achieved by transmission electron microscopy (TEM) analysis with a size range observed between 15 and 25 nm. Imidazolium based IL having longer alkyl chain behaves as a surface active agent (surfactant)67 and it is utilized to reduce

Ag+ ions forming AgNPs. DETA also reduces Ag+ ions and gives routine precipitation (PPT) of silver. Since IL is present in the reaction mixture, Ag+ ions are rapidly reduced to AgNPs accompanied by altered wavelength maxima, intensity of the absorption band and color of the solution. The imidazolium based IL forms micelles that support in delocalization of DETA molecules close to the micellar solution boundary due to the contact of DETA π-electrons with the headgroup of cationic imidazolium ring of IL (Scheme 1). Subsequently, imidazolium based IL forms micelles and adsorbed on AgNPs, furthermore changing their face by sinking or attracting the degree of particle aggregation relying on molecular constitution and concentration for the IL used. The CMC value for IL [Bmim][OS] was obtained from the plot of the absorbance versus [Bmim][OS] concentration (Figure 2). The observed CMC result is shown in Table 1. Figure 2 displays the UV−vis spectra for silver nanoparticles at various concentrations of [Bmim][OS]. In each of the above cases, a single surface plasmon absorption peak was seen in the visible region between 419 and 425 nm in the presence of [Bmim][OS]. The achieved absorption spectrum of AgNPs colloidal solution is seen to be parallel with the previous details for the AgNPs reported.68 The dissociation of AgNO3 into Ag+ ion and NO3− ion takes place in solutions. Correspondingly, NO3− ion resided into the micelle−water interface via 11091


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+DETA. A similar effect was noticed58 when AgNO3 (0.6 mM) was added in the synthesis of NPs; the CMC value of [Bmim][OS] reduces from 30 to 14 mM. It is obvious from Figure 3A that the CMC of [Bmim][OS] within the AgNPs +DETA system is observed to be less as compared to that in pure water. It is because of an increased number of Ag+ ions in the solution that diminishes the electrostatic repulsion between the negatively charged head groups of IL [Bmim][OS], thus facilitating the micelle formation and making the formation of aggregate structures energetically more constructive, therefore resulting on the micelles formation in relatively lower concentration. The interaction of NPs takes place with the headgroup of IL micelles, which is negatively charged and lowers the CMC of the IL. The standard Gibbs free energy of micellization (ΔG°m) has been calculated using eq 1:

Table 1. Critical Micelle Concentration (CMC) Values Achieved for 1-Butyl-3-methylimidazolium Octylsulfate via AgNPs Probe Response Using UV−vis Spectroscopy at Different Temperatures Temperature (K)

CMC (mM)

293 297 301

16 20 21

Coulomb force of attraction toward imidazolium cations of the positively charged [Bmim][OS] micelles. As a result, a bathochromic shift of the SPR band is noticed from 419 to 425 nm and further becomes unchanged (Figure 2A).69,70 Conductivity. Conductivity of the surface active imidazolium based IL was studied within AgNPs+DETA solutions. Specific conductivity against concentration plots of aqueous [Bmim][OS](mM) are shown in Figure 3A. A typical behavior is observed in Figure 3A, which can be illustrated by the presence of two linear regimes (pre and post) with dissimilar slopes. The conductivity can be linearly connected to the concentrations of IL [Bmim][OS] in pre- and postmicellar regions, where the observed slope in premicellar region is larger than that in the postmicellar regions.71 The meeting point of the two straight curves gives the CMC.72 It is noticed that conductivity is enhanced with the concentration of IL [Bmim][OS] as a result of a growing number of free ions in the solution as no micelle is present in the solution initially. Above the CMC, the increase in conductivity is lowered due to the formation of micelles and hence reducing the mobility of the ions in the solution. The CMC value of [Bmim][OS] solution observed in our study is seen to be close with the literature value.72 The obtained CMC values for pure [Bmim][OS] were 30 and 14 mM in the presence of AgNPs

ΔG°m = (2 − α)RT ln XCMC

(1)

where XCMC is the CMC in mole fraction unit. In both systems, the value of ΔG°m is negative, which signifies that the process of micellization is spontaneous. The ΔG°m value of all the systems in the presence of AgNPs+DETA was calculated on the basis of eq 1, and the results are listed in Table 2. Gibbs energy of transfer (ΔG°trans) as given in eq 2 was used to see the effect of AgNPs+DETA in the micellization behavior of IL [Bmim][OS]: ΔG°trans = ΔG°m(mixed media ) − ΔG°m(pure water)

(2)

Calculated values of ΔG°trans are given in Table 2. The observed negative ΔG°trans can be clearly visible from the reduction in the intermolecular interactions owing to the enhanced solvation, resulting in increased solubility of the hydrocarbon tails in the AgNPs+DETA system. Hence, a decrease in the CMC is noticed. It is eminent that the ΔG°trans

Figure 3. [A] Graph of specific conductance (κ) vs concentration of [Bmim][OS](mM) in the presence of AgNPs+DETA in aqueous media at 298 K. [B] Aggregate size distribution achieved from DLS at 298 K [A] AgNPs-DETA, [B] AgNPs-DETA-[Bmim][OS]. 11092


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Table 2. Critical Micelle Concentration (CMC), the Gibbs Free Energy of Micellization (ΔG°m), the Gibbs Energy of Transfer (ΔG°trans), the Gibbs Energy of Micellization Per Alkyl Tail (ΔG°tail), Degree of Dissociation (α) and Counterion Binding (β) for Ionic Liquid 1-Butyl-3-methylimidazolium Octylsulfate in the Presence of Nanoparticles/DETA at 298 K Medium

CMC (mM)

α

β

XCMC

ΔG°m kJ/mol

ΔG°trans kJ/mol

ΔG°tail kJ/mol

Water AgNPs+DETA

30.0 17.5

0.63 0.72

0.37 0.28

5.45 3.23

−18.95 −25.00

− −6.04

−9.47 −12.49

Mean errors in CMC = 0.1 mM, α = ±0.05, β = ±0.05, ΔG°m = ±1.0 kJ/mol, ΔG°trans = ±0.5 kJ/mol, ΔG°tail = ±1.0 kJ/mol.

a

Figure 4. DRS-FT-IR spectra [A] AgNP-DETA, DETA, AgNPs, [B] [Bmim][OS], AgNP-DETA-[Bmim][OS], AgNP-DETA.

Gibbs free energy transferred of the hydrophobic micellar core. This contribution is dependent on the transfer of Gibbs free energy of pure water and the interaction between AgNPs or DETA with IL due to the solvophobic effect. Dynamic Light Scattering. DLS is a simple and wellknown technique that is sensitive to small amounts of bulky particles from either from aggregation or infectivity, and this effect has been studied by DLS measurements.59 Figures 1C and 3B indicate the intensity for hydrodynamic diameter of the synthesized AgNPs (0.1 M) in aqueous IL [Bmim][OS] solution under ambient conditions. Polymodal distribution is observed for synthesized AgNPs (size 75.43, 15.77 and 1.33 nm) and in the presence of DETA. Observed sizes for NPs are 47.23, 1.921 and 5.01 nm, but in the presence of the [Bmim][OS]-DETA mixture bimodal size distribution is observed with particle sizes of 67.11 and 14.86 nm (Table S1), respectively. IL [Bmim][OS] seems to have a critical role

is mostly responsible for the increased tendency in the micellization of IL [Bmim][OS] in the mixed system, and the observed values rely on the transfer of Gibb’s free energy from water and the AgNPs+DTEA mixture solutions as well as their reciprocal interaction. As the mixture of AgNPs+DTEA changes the bulk phase making it more favorable than neat water for IL [Bmim][OS], the transfer of the hydrophobic tail from the bulk phase to the micellar region becomes less favorable. Hence, the ΔG°m value increases. The Gibbs free energy of micellization per alkyl tail is calculated as reported by eq 3: ΔG°m,tail = ΔG°m /2

(3)

Table 2 illustrates the negative ΔG°m,tail value for [Bmim][OS], which is lower as compared to that for the [Bmim][OS]+DETA binary mixture. While, the IL tail part transmits to the ΔG°m due to the IL interaction with DETA and also 11093


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copy. Interaction for IL toward BSA/HSA at various IL concentration regimes within the solution was thoroughly studied. The results obtained from different spectroscopic investigation provide insight into the unfolding of HSA/BSA as an effect of [Bmim][OS] binding. The corresponding outcomes achieved from the advanced multispectroscopic methods show extremely useful information on the mechanism of interaction involving IL and proteins. We have investigated the structural stability of protein (HSA/BSA) in the presence of IL [Bmim][OS]. Our results obtained from the aforementioned techniques are well supported by literature data. Kuriakose et al.77 synthesized AgNPs using the sol−gel process and these NPs were stabilized by encapsulation into the cavity of BSA protein. AgNPs and encapsulated materials were studied by SEM, TEM, XRD, FT-IR and NMR techniques. AgNPs encapsulated BSA proves to be highly efficient for antibacterial activity to bacterial strains, i.e., Serratia marcescens, Klebsiella pneumonia, Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli. Significant changes in the structural behavior of protein (HSA/BSA) were also earlier observed in literature data signifying that the neighbor surroundings of globular protein (HSA/BSA) were severely altered in the presence of ILs with the Hofmeister series of anions.78,79 UV−visible Spectroscopic Studies. UV−vis spectroscopy is the most constructive technique toward revealing information on the structural modification of proteins and studying protein−ligand complexation.80,81 The absorption spectra with various concentrations of IL [Bmim][OS] in AgNPs-DETA solution are given in Figure 5. The BSA/HSA

in changing the features of AgNPs. Modification in the micellar aggregates within the [Bmim][OS]-AgNPs system is clearly observed via electrostatic attraction in the presence of DETA (Scheme 1). As clarified earlier, electrostatic attraction among NO3− anion and AgNPs moieties with the micelles resulting in relatively larger micellar size. In the DLS results, the understanding of the major peaks is referred to as the hydrodynamic diameter of the AgNPs under a given set of experimental conditions. As reported by Mie’s theory,73 spherical AgNPs will present a single absorption peak; however, anisotropic AgNPs may give more bands. Eisa et al.73 studied cellulose nanocrystals (CNC) with silver and gold NPs. SPR and X-ray powder diffraction (XRD) results demonstrated the successful structure of the silver and gold (Au) NPs. FT-IR spectroscopy and electron microscopy results indicate stronger H-bonding among CNC within Ag and Au NPs and showed that the Ag and Au precursors created NPs of hexagonal and spherical character. Further, it is noticed that the wavelength corresponding to the SPR band maxima is sensitive to the dielectric properties as well as the size of the metal NPs and is seen to shift to greater wavelengths upon aggregation.74 Mogensen et al.74 studied the size dependent plasmon band of AgNPs films in aqueous solution. UV−vis absorption spectroscopy was used successfully for measuring the plasmon band position. Fourier Transform Infrared Spectroscopy. FT-IR is often used to verify which molecular structures are present in an object. DRS and attenuated total reflection (ATR)-FT-IR are most frequently utilized techniques in the study and recognition of inorganic or organic compounds such as nanoparticles, ILs, surfactants, resins, starch and proteins all of which are used in the self-assembly process.75,76 FT-IR uses an interferometer to modulate the wavelength from a broadband infrared source. AgNPs-[Bmim][OS]-DETA. FT-IR spectral responses are successfully utilized to classify the potential of biomolecules reliable as capping and reducing agents for the AgNPs synthesized. The two evident IR bands are observed at 3271 and 1637 cm−1. FT-IR spectra (Figure 4) of IL [Bmim][OS] show symmetric and asymmetric stretching CH2 vibration of alkyl chains at 2856.19 and 2926.19 cm−1 and are transferred to 2854.67 and 2925.92 cm−1, symmetric and asymmetric stretching of C−H scissoring vibration of CH3-moiety at 1466.13 cm−1 is transferred to 1466.18 cm−1, symmetric SO stretching vibration at 982.77 cm−1 transferred to 986.68 cm−1 in the [Bmim][OS]-AgNP-DETA system. The CC symmetric stretching at 1167 cm−1 is transferred to 1169.12 cm−1, CN symmetric stretching at 1058.31 cm−1 is transferred to 1065.12 cm−1, cis =CH out-of-plane bending at 752.18 cm−1 is transferred to 753.96 cm−1, aromatic −CC stretching at 1573.21 and 1581.97 cm−1 is transferred to 1572.09 cm−1. The development for hydrogen bonding strength depicts that [Bmim][OS] molecules mutually assembles in the aggregates and supports the dissociation of headgroup counterions in the surface of aggregates ensuring a closer arrangement of micelles (Figure 4). Interaction of Ionic Liquid-Silver Nanoparticles Mixture with HSA and BSA. Structural modification of our studied proteins, i.e., HSA and BSA, consequently of significant interaction with imidazolium based IL [Bmim][OS] have been explored using different spectroscopic methods viz., fluorescence, UV−visible, DLS, FT-IR and 1H NMR spectros-

Figure 5. UV−visible spectral response of [A and B] BSA/HSA in absence and presence of AgNPs, DETA, [Bmim][O]. [C and D] Benesi−Hildebrand plots using changes in absorption spectra of AgNP with serum albumins at 298 K.

protein gives an absorption peak at about λmax = 280 nm due to the existence of aromatic tyrosine, tryptophan and phenyl alanine amino acids. On adding IL [Bmim][OS], DETA, [Bmim][OS]-DETA, AgNPs, AgNPs-DETA and AgNPsDETA-[Bmim][OS], the peak intensity at 280 nm is gradually increased (Figure 5). The increase in absorbance shows the 11094


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Figure 6. Variation of size distribution accomplished from DLS at 298 K, respectively: [A] HSA-AgNPs-DETA-[Bmim][OS], [B] BSA-AgNPsDETA-[Bmim][OS].

M−1 for HSA at 298 K, from which protein−[Bmim][OS]/ NPs complexes infer that HSA shows more binding affinity compared to BSA. This may be due to the fact that HSA amino acid residues become more exposed within the IL−AgNPs system leading to stronger binding than compared to BSA. Fluorescence Studies. The fluorescence technique can be used to analyze the aggregation number, binding constants, Stern−Volmer quenching constants of the complexes and also the interaction between the macromolecules and metallic nanoparticles.80 In our work, fluorescence spectra for globular proteins with IL [Bmim][OS] in AgNPs solutions are used to verify the complex formation. IL AgNPs or DETA shows no fluorescence within the wavelength range between 200 and 600 nm, so their spectral contribution can be ignored. [Bmim][OS] concentrations taken in our study for the calculation of binding constants (Ka) are comparable to the HSA/BSA concentration. The fluorescence band due to HSA/BSA was observed at λmax = 340 nm. It showed a decreased intensity at 340 nm because of a complexation event among HSA/BSA with [Bmim][OS] (Figure S2). The binding constant (Ka) was obtained from Eq 5: ÄÅ É Å F − F ÑÑÑ ÑÑ = log K a + n log[Bmim][OS] logÅÅÅÅ 0 ÅÅÇ F ÑÑÑÖ (5)

formation of a complex between the globular proteins and the AgNPs/[Bmim][OS]/DETA system. Chen and his group82 studied the interaction between 4-acetyl-N-propenylpyridinium chloride based tailored magnetic Fe3O4 (Fe2) and BSA then compared the results with amine (ANH2) functionalized magnetic NPs Fe 3O 4 (Fe1) based upon spectroscopy techniques. The results show a static quenching mechanism working in two NPs via formation of the Fe2−BSA complex. BSA protein has shown a weak binding affinity for Fe2 than Fe1. In this study, the UV−vis absorption spectrum is a useful tool to probe protein−IL complex formation in the presence of AgNPs/DETA. Since [Bmim][OS] and DETA have almost no UV−vis absorption in the wavelength range 200−600 nm, their spectral absorbance can be neglected. The [Bmim][OS] concentrations used for the evaluation of the binding constants were comparable to the BSA/HSA concentration. The UV−vis absorption due to BSA/HSA exhibited band near λmax=280 nm (Figure 5). The binding constant of the protein [Bmim][OS] and NPs complex is determined using Benesi−Hildebrand eq 4. 1 1 1 = + A − A0 K [A max − A 0][Bmim][OS] A max − A 0 (4)

where Ka is the binding constant of [Bmim][OS] to HSA/BSA and n number of binding sites. Ka and n can be found from the plot of log[(F0 − F)/F] against log[Bmim][OS] using the least-squares algorithm (Figure S2). They are found to be statistically similar binding constants to those obtained from UV−vis spectroscopy. Kothari et al.83 have studied the BSA− AgNPs interactions using fluorescence spectroscopy. The fluorescence results signified that AgNPs show strong efficiency to quench the intrinsic fluorescence of BSA via static and dynamic quenching pathways.

where A0 and A are UV−vis absorbance of BSA/HSA in the absence and presence of IL, respectively, Amax is the absorbance at intermediate concentration of IL [Bmim][OS] saturated and K is the binding constant. Plots for 1/[A − A0] provide straight lines (Figure 5C,D), which shows the 1:1 stoichiometric complex formation among protein and [Bmim][OS]/NPs mixture. The values for the binding constant that is observed from the intercept-to-slope ratio in Benesi−Hildebrand plots is found to be 1100.09 ± 2.0 M−1 for BSA and 1628.80 ± 2.0 11095


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Figure 7. DRS-FT-IR spectra HSA, HSA+[Bmim][OS] and HSA+[Bmim][OS]+AgNPs+DETA.

Table 3. Chemical Shift Value of 1-Butyl-3-methylimidazolium Octylsulfate and Human Serum Albumin (HSA), Bovine Serum Albumin (BSA) with [Bmim][OS]-AgNPs Mixture Proton [Bmim][OS] HSA Complex BSA Complex

H1

H2

H3

H4

1.253, 1.235, 1.216, 1.519, 1.159 1.236, 1.218, 1.200

1.546, 1.531 1.739

1.856, 1.786, 1.768 1.759

4.162, 4.123, 4.106, 4.088 4.151

1.239, 1.220, 1.201

1.257

1.276

4.123, 4.105, 4.087

Dynamic Light Scattering. Here, we have used DLS for AgNPs particle size distribution and AgNPs−protein interface study. In the size-distribution diagram, AgNPs exhibit polymodal size distribution peaks with average hydrodynamic diameters at 75.43, 15.77 and 1.33 nm. Figure 6 reveals the scattering intensity of the hydrodynamic diameter (Rh) of solutions with two globular proteins (BSA and HSA) with the IL [Bmim][OS] under ambient conditions. A bimodal distribution is found for HSA in the AgNP-[Bmim][OS]DETA mixture. The average radius of HSA shows a polymodal distribution with a larger Rh = 57.17, 3.297, 7.356 (d. nm), PDI = 0.175 and Z-average (d. nm) = 57.75. The average radius of BSA shows a bimodal distribution with larger Rh = 63.01, 5.883, 5129 (d. nm), PDI = 0.641 and Z-average (d. nm) = 41.07 (Table S1). Those outcomes show that globular proteins form aggregates with AgNP/IL/DETA and the proteins-AgNP/IL/DETA complexes are formed. In the entire study, the data explains the micelle-like structure formation even in a NPs mixture. Fourier Transform Infrared. The IR spectra of AgNPs (Figure 1C) dispersed with HSA/BSA (Figure 7 and S1) were recorded using a KBr pellet in the scanning range between 4000 and 450 cm−1. The interaction of the globular proteins with AgNPs in the presence of [Bmim][OS]-DETA using FTIR is shown in Figure 7. Proteins revealed two absorbing bands, amide I band at 1644 cm−1 (CO stretching) and amide II band at 1540 cm−1 (CN stretching and NH bending mode). The peak site of the amide I band is shifted from 1644 to 1657 cm−1. Moreover, the amide II band shifted slightly from 1540 to 1546 cm−1. These results express that [Bmim][OS] also affects the stretching vibration of CO groups in the protein polypeptides along with the CN

H5

H6

3.936, 3.921, 3.905 4.103, 4.085, 4.067 3.799

H7

H8

8.765, 8.753 8.372

7.394

7.351

7.335

7.380

8.043

7.336

7.382

stretching and NH bending vibration modes. Generally, the interaction of [Bmim][OS] with globular proteins resulted in the rearrangement of the serum albumins secondary structure and [Bmim][OS] at larger concentration shows greater influence on the structure change of the serum albumins which corroborate well with the results observed from fluorescence, UV−vis and DLS experiments. Wang et al.84 studied the interaction of BSA and silver nanoparticles using various spectroscopic methods. Results show the interaction of AgNPs and BSA complex and they have also determined the thermodynamic parameters and binding constants. Wu et al.85 studied the tetrabutylphosphonium styrenesulfonate ([P4,4,4,4][SS]) and BSA interaction using different spectroscopy methods. The changes in the frequency observed from FT-IR outcomes indicate the conformational changes of random coil, α-helix to β-turn and β-sheet with no intermediate transition state formation. Nuclear Magnetic Resonance. NMR is also a powerful tool for investigation of the macromolecular interaction, complex forming and characterization. Hence, NMR has been widely applied to explore the structure of macromolecules, ILs and complexation between metallic nanoparticle and globular proteins, drugs and various applications in pharmaceutics. During the interaction, the small molecule hydrogen that is in close contact with the AgNPs obtained the most intense chemical shift as well as most intense NMR signals. Both globular proteins HSA and BSA are highly soluble to make the NMR phenomenon obvious; the DETA−IL mixture in AgNPs and proteins were thoroughly studied. The best results were obtained for the IL−HSA mixture. The NMR experiments were done using a 400 MHz Bruker NMR spectrometer. The observed results can provide suitable 11096


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spectroscopy, conductivity and more interestingly, the present results are superior and more revealing. This study on the interaction between globular proteins and the AgNPs/IL [Bmim][OS] system can be potentially applied in several fundamental and applied interdisciplinary fields, notably in nanoscience, biocatalysis, biosensing and pharmaceutics.

evidence on the effect of added imidazolium based IL in the formation from complex with globular proteins in water solution. In present study, the effect of IL [Bmim][OS] on the BSA/HSA complexation was studied and the chemical shifts of [Bmim][OS] IL protons were measured. Figure S3 shows the proton NMR spectra of 0.1 M [Bmim][OS] in D2O solution with HSA/BSA. It can be seen that after adding both globular proteins, the 1H signals of H6, H7 and H8 of the imidazolium ring show downfield shifts and H5 of the methyl group on the imidazolium cation shows an upfield shift and H1 to H4 in the alkyl chain show upfield shifts. All 1H NMR data are shown in Table 3. In the present investigation, we have focused on the NMR chemical shift values of all protons for IL [Bmim][OS] in the presence of HSA and BSA serum proteins. The effect on both HSA and BSA resonating protons has also been observed in the presence of [Bmim][OS] and AgNPs mixture; chemical shift data are presented in Table 3. From Figure S3, it is clearly classified that the chemical shifts of HSA/BSA in the presence of IL [Bmim][OS] show significant changes. This leads to a clear indication that HSA and BSA have an affinity toward [Bmim][OS]. However, when globular proteins (HSA/BSA) are added to the mixture of AgNP with IL (Figure S3), chemical shift values change significantly. The presence of residual water signals (4.7 ppm) characterizes that water interacts with the polar surface of HSA and BSA macromolecules. Signals of HSA at 3.798 ppm and BSA at 3.799 ppm, which were not properly resolved in 1H NMR as a result overlapping peaks for [Bmim][OS], are now well resolved (Figure S3). The peak due to water molecules present in D2O is also observed in the spectra (Figure S3) at 4.701 ppm. Kumar et al.86 studied the 1-butyl-3-methylimidazolium alkyl sulfate, with two distinct model proteins, namely HSA and collagen with the help of solution NMR, T1SEL and T1NS measurements that show the genuine protein−IL binding attraction that is improved with an expansion in carbon chain length with the anionic portion of the IL.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06598. Figure S1. DRS-FT-IR spectra [A] pure HSA, BSA and mixture of HSA+[Bmim][OS], BSA+[Bmim][OS], [B] BSA+AgNP+[Bmim][OS]+DETA. Figure S2. [A] Fluorescence Spectra of HSA in the presence of various concentration of ionic liquid [Bmim][OS] and Stern− Volmer plots of [B] BSA-AgNP+[Bmim][OS]+DETA and [C] HSA+AgNP+[Bmim][OS]+DETA. Figure S3.1H NMR spectra of [A] pure [Bmim][OS], [B] HSA-[Bmim][OS]-AgNP-DETA, [C] BSA-[Bmim][OS]-AgNP-DETA. Table S1. Hydrodynamic radii (Rh) and polydispersity of human serum albumin and bovine serum albumin incubated at different conditions. Scheme S1. Molecular structure of ionic liquid 1-butyl-3methylimidazolium octylsulfate, diethylene triamine, bovine serum albumin and human serum albumin. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-771-2263146 (O). Fax: +91-771-2262583. E-mail: [email protected]; [email protected] (K. K. Ghosh). ORCID

Siddharth Pandey: 0000-0003-3175-5353 Kallol K. Ghosh: 0000-0002-5805-0305

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CONCLUSIONS In conclusion, we have studied the interaction between AgNPs and environmentally benign IL 1-butyl-3-methyl imidazolium octylsulfate [Bmim][OS] in the presence of DETA. The CMC is observed via in situ AgNPs formation, which shows a novel way of CMC determination and in addition an entire new synthesis method for AgNPs. To the best of our knowledge, this is the first report of CMC determination of short-chain imidazolium based IL [Bmim][OS] reported on the basis of SPR band shift and color changes of AgNPs. The IL−protein binding affinity of [Bmim][OS] toward albumin proteins HSA and BSA is studied using various spectroscopic methods viz., fluorescence, UV−vis, DLS, FT-IR and 1H NMR. From UV− vis and fluorescence spectral results, it is observed that IL [Bmim][OS] shows stronger binding affinity toward HSA as compared to BSA. This may be due to the fact that the amino acid residues of HSA are slightly more exposed toward the IL− AgNPs system than compared to BSA and results in relatively stronger binding. This was further confirmed by DLS, FT-IR and 1H NMR spectroscopic results. It is noteworthy to mention that the current results on IL−protein interactions corroborate with the results obtained from our earlier reported works80,81 on the interaction of proteins with fluorescent carbon dots and monomeric and dimeric surfactants studied via fluorescence, UV−vis spectroscopy, surface tension, FT-IR

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manoj Kumar Banjare gratefully acknowledges fellowship received from the Pt. Ravishankar Shukla University, Raipur (C.G.). Dr. Kamalakanta Behera is gratefully acknowledged to the Science and Engineering Research Board (SERB), New Delhi, India, for providing a research grant (YSS/2015/ 001997). The authors are grateful to National Center for Natural Resources, Pt. Ravishankar Shukla University, Raipur (C.G.) for providing the nuclear magnetic resonance and Prof. S. Saraf, School of Studies in Pharmacy, Pt. Ravishankar Shukla University, Raipur (C.G.) for providing the experimental facilities to measure the aggregate size distribution obtained from DLS. We also acknowledge Prof. M. K. Deb, Head, School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur (C.G.) for providing the FT-IR and laboratory facility.

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Interaction of Ionic Liquid with Silver Nanoparticles: Potential

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