Accepted Manuscript Bio-functionalized silver nanoparticles for selective colorimetric sensing of toxic metal ions and antimicrobial studies V. Vinod Kumar, S. Anbarasan, Lawrence Rene Christena, Nagarajan SaiSubramanian, Savarimuthu Philip Anthony PII: DOI: Reference:
S1386-1425(14)00431-4 http://dx.doi.org/10.1016/j.saa.2014.03.020 SAA 11853
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
23 December 2013 28 February 2014 15 March 2014
Please cite this article as: V. Vinod Kumar, S. Anbarasan, L.R. Christena, N. SaiSubramanian, S.P. Anthony, Biofunctionalized silver nanoparticles for selective colorimetric sensing of toxic metal ions and antimicrobial studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.03.020
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Bio-functionalized silver nanoparticles for selective colorimetric sensing of toxic metal ions and antimicrobial studies V. Vinod Kumar, S. Anbarasan, Lawrence Rene Christena, Nagarajan SaiSubramanian and Savarimuthu Philip Anthony* School of Chemical & Biotechnology, SASTRA University, Thanjavur-613401, Tamil Nadu, India. Fax: +914362264120; Tel: +914362264101; E-mail:
[email protected]
Abstract Hibiscus Sabdariffa (Gongura) plant extracts (leaves (HL) and stem (HS)) were used for the first time in the green synthesis of bio-functionalized silver nanoparticles (AgNPs). The bio-functionality of AgNPs has been successfully utilized for selective colorimetric sensing of potentially health and environmentally hazardous Hg2+, Cd2+ and Pb2+ metal ions at ppm level in aqueous solution. Importantly, clearly distinguishable colour for all three metal ions was observed. The influence of extract preparation condition and pH were also explored on the formation of AgNPs. Both selectivity and sensitivity differed for AgNPs synthesized from different parts of the plant. Direct correlation between the stability of green synthesized AgNPs at different pH and its antibacterial effects has been established. The selective colorimetric sensing of toxic metal ions and antimicrobial effect of green synthesized AgNPs demonstrated the multifunctional applications of green nanotechnology.
Keywords: Biosynthesis of AgNPs, AgNPs colorimetric sensor, Cd2+ sensor, Pb2+ sensor.
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1. Introduction Nanostructured noble metals which exhibit unique optical and optoelectronic properties in the size range of 1–100 nm have found widespread applications in modern technologies [1]. Several synthetic approaches have been employed to control size, morphology and crystallinity to meet the specific demands of different applications [2]. Of late, there has been of considerable interest in utilizing green chemistry principles to produce noble metal nanoparticles with controlled size and morphology [3]. Green chemistry is the design, development, and implementation of chemical products and processes to reduce or eliminate the use and generation of substances hazardous to human health and the environment [4]. The wet chemical approach which is the most widely used in the synthesis of metal and semiconductor nanostructures generally involves the usage of environmentally hazardous and potentially toxic chemicals such as hydrazine, surfactants and organic solvents in the reactions [5]. Green nanotechnology that utilizes naturally occurring molecules as reducing and capping agents has provided alternate pathway to synthesis stable NPs [6]. Particularly the use of multifunctional and environmentally friendly materials has evoked enhanced interest in the recent years. For example, tea/catechin is the most widely used behaviorally active drug in the world and has high water solubility, low toxicity and biodegradability. The chemical molecules present in the tea also successfully serve the function of reducing and capping agent for noble metal nanoparticles [7]. Further silver ions are the most widely used for the antimicrobial treatment of critical burn wounds [8]. In green nanotechnology, it was expected that the inclusion of plant extracts with AgNPs would improve the antimicrobial effects [9].
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Selective and ultrasensitive detection of potenetially toxic metal ions present in the environment or biological samples becomes increasingly important [10]. For example, mercury is one of the most prevalent toxic elements in the environment and poses serious health threats because of its high affinity for thiol groups in proteins and enzymes, thereby leading to the dysfunction of cells and consequently causing many health problems in the brain, kidney, and central nervous system [11]. Similarly lead is listed second in the list of toxic substances in the environment due to its wide distribution and use in batteries, gasoline and pigments [12]. Even low level exposure of lead can lead to neurological, reproductive, cardiovascular and developmental disorders [13]. Children with variants in iron metabolism genes may be more susceptible to lead absorption and accumulation. Cadmium is also known as a toxic metal ion although it is inessential to life, it can cause serious diseases, such as renal dysfunction, disorders in calcium metabolism disorders, prostate cancer, etc [14]. Although various techniques are available to monitor the contamination, colorimetric based sensor has received strong interest due to their simplicity, rapidity, high sensitivity and ease of measurement that also allows online, real-time analysis [15]. Particularly, noble metal NPs such as AgNPs that exhibit unique optical properties, excellent stability, good biocompatibility and water solubility could be a potential candidate to fabricate colorimetric sensor [16,17].
The
surface
functionalization of AgNPs with appropriate metal interacting chemical unit controls the selective sensing of heavy metal ions. For example, AgNPs appended with different chemical functionalities have been used as analytes for Ni2+, Co2+, Hg2+, Pb2+ and Cu2+ [18]. Amino acid attached phenolic chelating ligands have been shown as potential reducing and surface functional agent in the fabrication of AgNPs colorimetric sensor for Cd2+ and Pb2+ [19]. Plant extracts that contains metal
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interacting multi-functional groups such hydroxyl, carboxyl and hetero-aromatic rings [20] offers an excellent opportunity to develop environmentally benign and costeffective AgNPs colorimetric sensor for potentially hazardous metal ions [21]. Although green synthesis of AgNPs from plant extracts and its antimicrobial studies, cytotoxicity and biocompatibility studies are well reported, [6,8] studies on other applications such as heavy metal ions sensing are rarely reported. Herein, we report the green synthesis and colorimetric sensing of biofunctionalized AgNPs from Hibiscus Sabdariffa plant extracts in presence of sun light for the first time to our knowledge. Green synthesized AgNPs showed selective colorimetric sensing of potentially health and environmental hazardous Hg2+, Cd2+ and Pb2+ metal ions in aqueous medium with distinguishable colour. It was observed that the selectivity and sensitivity of AgNPs depend on the parts of plant extracts (HL and HS) used. Similarly AgNPs at different pH (3.2, 7.0 and 10.5) displayed different antibacterial effects. A correlation between green synthesized AgNPs stability and antibacterial effects has also been established. 2. Experimental methods Fresh Hibiscus Sabdariffa was purchased from a shop in Thanjavur, India. Both leaves and stems were separated for nanoparticles preparation. AgNO3 was obtained from Sigma Aldrich and used as received. Milli-Q water was used for all the experiments. All heavy metal salt solutions were prepared by dissolving requisite amount of salt in Mill-Q water. The experiments were performed directly under sun in the daylight between 11 am to 3 pm in the month of March to May-2013. Each experiment was repeated three times to confirm the reproducibility.
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2.1. Preparation of AgNPs from Hibiscus Sabdariffa leaf extracts (HL-AgNPs) Typically, fresh Hibiscus Sabdariffa leaves (10 g) were washed with milli-Q water and air dried. Then it was divided into two equal parts. One part (5 g) was soaked in 50 ml of milli-Q water for 60 min under room temperature (HLa) whereas another part (5 g) was soaked in 50 ml of milli-Q water for 60 min under hot condition (HLb, 60 °C). Then both the extracts were filtered using Whatman filter paper. Further each batch of extracts was divided into three 5 ml portion with different pH (initial pH: 3.2 (HL1a, HL1b), pH: 7.0 (HL2a, HL2b) and pH: 10.5 (HL3a, HL3b)). The pH was adjusted by adding 0.01 M NaOH. Aqueous solution of AgNO3 (3 ml, 10-3 M) was added into the 5 ml extract solution (HL) under stirring at room temperature. The solution was stirred for another 30 min. The colourless solutions were converted into pale pink to intense brownish-yellow colour within 10 min upon exposure to sunlight. The change of colour confirms the formation of HLAgNPs. The solution was stored at room temperature for two days before doing any analysis. 2.2. Preparation of AgNPs from Hibiscus Sabdariffa stem extracts (HS-AgNPs) Freshly cut Hibiscus Sabdariffa stem (10 g) was washed with milli-Q water and air dried. Extracts from stem was prepared similar to previous procedure at room temperature (HSa) as well as under hot condition (HSb, 60 °C). Further it was divided into three different pH solutions (4.0, 7.0, 10.5; HS1a-HS3a, HS1b-HS3b). Aqueous solution of AgNO3 (3 ml, 10-3 M) was added into the 5 ml of extract solution (HS) under stirring at room temperature. The solution was allowed stir for another 30 min. The colourless silver nitrate solution was converted into pale pink to intense brownish-yellow colour within 10 min upon exposure to sunlight. The conversion
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colourless to strong colour confirms the formation of HS-AgNPs. The solution was stored at room temperature for two days before doing any analysis. 2.3. Characterization The UV-visible measurement of the green synthesized AgNPs were analyzed in a Perkin Elmer model UV–Vis double beam spectrophotometer. Zeta potential measurements were carried out using a Zetasizer ver.6.20. The aqueous suspension of AgNPs was taken in a cuvette. Zeta potential is measured by the principle of Electrophoretic mobility created by applying an electric field across the dispersion media. AgNPs morphology and sizes were analyzed using field emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL Japan INC) and High Resolution Transmission Electron Microscopy (HR-TEM, JEOL JEM-2100F operated at an accelerated voltage of 200 kV and an ultra high-resolution pole piece). FT-IR measurements were carried out on a Perkin–Elmer Spectrum-One instrument in the diffuse reflectance mode at a resolution of 4 cm-1 in KBr pellets. 2.4. Biological studies E.coli cultures were obtained from MTCC, Chandigarh, India. Cultures were maintained in the form of 10 % glycerol stocks in -70oC and were revived by routine sub-culturing on LB agar plates. The silver nanoparticles (HL-AgNPs and HSAgNPs) were tested for antibacterial activity by standard disc diffusion method. Muller Hinton Agar plates were seeded with 24 h broth culture of E.coli. Each strain was swabbed uniformly onto the individual plates using sterile cotton swabs. Wells of 10mm diameter were made on nutrient agar plates using gel puncture. Using micropipette, 50 - 200 µl of AgNPs solution was pipetted onto each well on all plates.
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After allowing for diffusion at room temperature for 2 h, the plates transferred to an incubation chamber maintained at 37 oC for 24 h. 3. Results and Discussion Hibiscus sabdariffa is well known in south Asia due to its medicinal properties [22]. Tender young leaves and stems raw or cooked are used in salads, as a pot-herb and seasoning in curries, they have an acid, rhubarb-like flavor. Hibiscus sabdariffa leaves helps to reduce inflammation and has anti-pyretic effect. Fresh leaves help to stimulate the stomach and to sharpen the appetite. It is said to have diuretic effects, anti-pyretic effects and is antiscorbutic. It is used as a folk remedy in the treatment of bilious conditions, cough, hangover, heart ailments, hypertension, and neurosis. In general, mixing silver nitrate solution with plant extracts at room temperature have shown to produce AgNPs with yellow to intense brown colour [9]. However, addition of AgNO3 (10-3 M, 3 ml) into HL and HS extract (5 ml) did not produce any significant colour change. The heating (30 min at 60 °C) also did not have any significant effect. But immediate colour change was observed in all samples (yellow or brownish yellow) upon exposure to sunlight and confirmed the formation of AgNPs (Fig. 1). Although it is difficult to predict the exact molecular component of extracts involved in reducing silver ions into AgNPs, the presence of tannins and other phytochemical molecules could be responsible for the conversion of silver ions into AgNPs [22a]. For example, polyphenolic compounds such as tannins are known to undergo photo-oxidation easily in presence of sunlight that would reduce silver ions into AgNPs [23, Scheme 1]. The absorption studies of HL1a-and HL1b-AgNPs showed broad absorption peak (λmax) from 625 to 470 nm (Fig. 2). A small hump was also observed for HL1b-AgNPs at 378 nm (Fig. 2). These absorptions are due to
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excitation of surface plasmon resonance (SPR) of noble NPs [24]. HL1a and HL1b did not show any absorption in the visible range. But it exhibits yellow colour with a clear absorption at 396 nm at higher pH (HL2, 7.0; HL3, 10.5) (Fig. S1). HL2aAgNPs showed absorption at 400 nm while HL3a-AgNPs showed red shifted absorption at 428 nm. Both HL2b- and HL3b-AgNPs exhibited absorption at 380 nm. Although HL2b and HL3b (without AgNPs) showed λmax around similar position, the solution colour and absorption λcut-off showed completely different. HS1a-AgNPs showed weak absorption peak around 450 nm but clear single broad absorption was observed at 460 nm for HS1b-AgNPs. However, HS2- and HS3-AgNPs showed only a single absorption at 390 nm and 403 nm, respectively (Fig. 3). Again, fresh extract, HS1a and HS1b did not exhibit any absorption in the visible range. The extracts at higher pH displayed light yellow colour and absorption spectra showed weak peak at 366nm (Fig. S1). The green synthesis of AgNPs was performed using extracts prepared at different temperature and pH because of the following reason. The extract obtained at higher temperature is expected to have larger amount phytochemicals because of enhanced solubility at higher temperature that might help in AgNPs formation. Different parts of plant extract were used with the intent of exploiting the possible chemical differences for AgNPs synthesis and sensor applications [21b]. Extract at different pH was utilized since deprotonated phenolates or carboxylates can provide better stability for AgNPs. The phenolates are known to reduce silver ions into AgNPs very effectively because of its higher ionization energy [25]. Although the chemical differences of leafs and stems are not clear, the Zeta potential measurement showed higher stability for AgNPs obtained from stems extract as well as prepared at higher pH [Table S1].
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The green synthesized AgNPs size and morphology were characterized by using FE-SEM and HR-TEM. The formation of spherical AgNPs with different sizes in HL- and HS-AgNPs was clearly evidenced from FE-SEM images (Fig. S2). HRTEM studies further confirmed the polydispersed spherical crystalline NPs formation in HL1a-AgNPs and HL2a-AgNPs (Fig. 4). HL1a-AgNPs that showed weak absorption band around 520 nm exhibited formation of different morphology with high polydispersity (30 to 100 nm) whereas clear spherical NPs in the size range of 5 to 30 nm was witnessed in HL2a-AgNPs (pH 7.0). Prominent IR bands were observed at 3434, 2931, 2853, 1696, 1615, 1430 and 1385 cm−1 which confirmed the presence of flavonoids, alkaloids and phenolic compounds in the plant extracts that acted as reducing as well as stabilizing agents (Fig. S3). The selective colorimetric sensing properties of HL- and HS-AgNPs were explored for a series of heavy metal ions in aqueous solution. The pale pink colored HL1-AgNPs showed high sensitivity towards all metal ions without any selectivity. It is noted that the pink colour was disappeared with all metal ions (Fig. S4). Absorption spectra of HL1-AgNPs also did not show any peak in the visible range with metal ions (Fig. S4). HL2a-AgNPs showed only Hg2+ sensing (Fig. 5) but HL2b-AgNPs showed naked eye detectable visible colour change for Hg2+, Cd2+ and Pb2+ metal ions (Fig. 6a,b). Concentration dependent studies showed that HL2b-AgNPs absorption peak was completely disappeared upon addition of 300 µl of Hg2+. Similarly, 420 µl of Cd2+ almost completely reduced HL2b-AgNPs absorption peak at 396 nm and a new peak appeared as hump at longer wavelength (506 nm, Fig. 6c). Similarly, reduction of absorption intensity (396 nm) was observed at initial Pb2+ additions, but the peak was completely disappeared at 260 µl additions with increased λcut-off to longer wavelengths (Fig. 6d). HL3-AgNPs did not show any selective colorimetric
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sensing at low concentration but it showed selective sensing of Hg2+ at higher volume (2000 µl, Fig. S5). The colorimetric sensing studies of HS-AgNPs with various metal ions showed only selective sensing of Hg2+. HS1-AgNPs showed complete colour disappearance upon addition of Hg2+ (Fig. S6). The concentration dependent studies revealed that HS1a-AgNPs required 200 µl of Hg2+ whereas HS1b-AgNPs needed 300 µl additions. HS2-AgNPs also showed selective decolourisation of yellow or brownish yellow colour with Hg2+ selectively but it required higher concentration of Hg2+. Concentration dependent studies of HS2-AgNPs with Hg2+ showed that the absorption peak intensity was only slightly reduced up to 520 µl addition (Fig. S7). HS3a-AgNPs did not show any noticeable colour change for any metal ions up to 4000 µl addition of Hg2+ (Fig. S8). The mechanism of Hg2+ sensing by green synthesized AgNPs could be explained based on the electrochemical differences of Ag+ and Hg2+ ions. The standard reduction potential for Ag is +0.80 V (Ag+ + e- = Ag) whereas for Hg2+ it is +0.92 V (2Hg2+ + 2e- = Hg22+) and according to the electrochemical series, metals with higher reduction potential acted as better oxidizing agents [21]. The differences in electrochemical reduction potential for the oxidation of AgNPs was further confirmed by another experiment with KMnO4 (+1.51 V, 10-4 M, 0.5 ml), a higher reduction potential metal ions, that also exhibited immediate decolourisation of yellow colour (Fig. S9). Selective colorimetric sensing of Cd2+ and Pb2+ with bathochromic shift of absorption is due to the interaction of surface functionality of NPs that leads to the formation of aggregates of AgNPs. HR-TEM studies clearly revealed the formation of aggregates by Pb2+ and Cd2+ with HL2b-AgNPs (Fig. S10). Theoretical and experimental studies has shown that the plasmon oscillation of metal
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nanoparticles induce them to couple to each other when they are brought in proximity [26]. Silver is known for its antimicrobial properties and has been used for years in the medical sciences [27]. HL- and HS-AgNPs were also tested for its growth inhibition studies against E.coli (Fig. 7). Plant extracts (HL and HS) alone were used as controls. HL1-AgNPs and HL2a-AgNPs clearly showed growth inhibition in E.coli. However, HL2b- and HL3-AgNPs did not show any growth inhibition. Surprisingly all HS-AgNPs did not show any antimicrobial effects (Fig. S11). It is interesting to note that AgNPs formed by different parts of same plant extracts showed completely different antimicrobial effects. Zeta potential measurements of HL- and HS-AgNPs were performed to get the insight of AgNPs stability prepared at different pH/condition (Table S1). The increasing negative potential of HL- and HSAgNPs samples with increasing pH indicates the enhanced AgNPs stability at higher pH. AgNPs prepared using stem extracts (HS-AgNPs) exhibited higher stability than HL-AgNPs. The selective decolourisation of AgNPs by Hg2+ is due to oxidation of Ag to silver ions. The concentration dependent studies of Hg2+ with AgNPs showed that samples with higher pH values required more amount of Hg2+. Similar results were obtained with KMnO4 that also oxidize AgNPs into silver ions (Fig. S8). The enhanced stability of AgNPs at higher pH might be possible via stronger capping by deprotonated phenols and carboxylic acid. These results suggest that the surface capping molecules plays a key role in the resulting applications such as antimicrobial activities and colorimetric sensor of AgNPs. 4. Conclusion We have demonstrated the green synthesis of bio-functionalized AgNPs using Hibiscus Sabdariffa plant extracts in presence of sunlight. The utility of
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biofunctionality of AgNPs were successfully used in selective colorimetric sensing of potentially health and environmental hazardous Hg2+, Cd2+ and Pb2+ metal ions at ppm level in water. The role of different parts of the plant and pH in the AgNPs formation and colorimetric sensor applications were also explored. The antimicrobial studies of AgNPs prepared using different parts of plant and different pH (3.2, 7.0 and 10.5) exhibited different effects. A direct correlation between green synthesized AgNPs stability and antibacterial effects were also demonstrate. These studies suggest that apart from its usual biological and biocompatible properties, bio-functionality of green synthesized AgNPs can also be used in environmental sensing applications. Acknowledgements Financial supports from Department of Science and Technology, New Delhi, India (DST Fast Track Scheme No. SR/FT/CS-03/2011(G) is acknowledged with gratitude. References [1] a
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Figure Captions Figure 1
Biosynthesis of AgNPs using HL and HS extracts in presence of sunlight.
Scheme 1
Mechanism of AgNPs formation from polyphenols under sunlight.
Figure 2
Absorption spectra of HL-AgNPs.
Figure 3
Absorption spectra of HS-AgNPs.
Figure 4
TEM images of (a) HL1a-AgNPs and (b) HL2a-AgNPs.
Figure 5
(a) Digital images with absorption spectra of HL2a-AgNPs with metal ions and (b) HL2a-AgNPs absorption Vs Hg2+ concentration.
Figure 6
(a) Digital images with absorption spectra of HL2b-AgNPs with metal ions and HL2b-AgNPs absorption Vs Hg2+ (b), Cd2+ (c) and Pb2+ (d) concentration.
Figure 7
Antibacterial studies of (i) HL1a (a), HL1a-AgNPs (b) HL2a-AgNPs (c), HL3a-AgNPs (d) and (ii) HL1b (a) HL1b-AgNPs (b), HL2bAgNPs (c) HL3b-AgNPs (d).
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Figure 1
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Scheme 1
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Figure 2
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Figure 3
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Figure 4
Figure 5
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Figure 6
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Figure 7
Graphical Abstract
Bio-functionalized silver nanoparticles for selective colorimetric sensing of toxic metal ions and antimicrobial studies
Bio-functionality of green synthesized AgNPs from Hibiscus Sabdariffa plant extracts were successfully utilized for selective colorimetric sensing of potentially health and environmentally toxic metal ions such as Hg2+, Pb2+, Cd2+ in aqueous solution.
Highlights
Green synthesis of AgNPs with metal ions interacting biofunctional surface functionality using Hibiscus Sabdariffa plant extracts for the first time.
Role of sunlight, pH and different parts of plant on the formation of AgNPs.
Selective sensing of potentially toxic Hg2+, Cd2+ and Pb2+ metal ions with distinguishable colour in aqueous solution.
Direct correlation between AgNPs stability and antimicrobial studies.
