Epicatechin coated silver nanoparticles as highly selective nanosensor for the detection of Pb2+in environmental samples

Accepted Manuscript Epicatechin coated silver nanoparticles as highly selective nanosensor for the detection of Pb2+in environmental samples

Farhat Ikram, Amtul Qayoom, Zara Aslam, Muhammad Raza Shah PII: DOI: Reference:

S0167-7322(18)33634-1 https://doi.org/10.1016/j.molliq.2018.12.146 MOLLIQ 10222

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

15 July 2018 23 December 2018 29 December 2018

Please cite this article as: Farhat Ikram, Amtul Qayoom, Zara Aslam, Muhammad Raza Shah , Epicatechin coated silver nanoparticles as highly selective nanosensor for the detection of Pb2+in environmental samples. Molliq (2018), https://doi.org/10.1016/ j.molliq.2018.12.146

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ACCEPTED MANUSCRIPT Epicatechin coated silver nanoparticles as highly selective nanosensor for the detection of Pb2+in environmental samples.

Farhat Ikram1, Amtul Qayoom1, Zara Aslam2, Muhammad Raza Shah2 1

Department of Chemistry, N.E.D. University of Engineering and Technology, Karachi 75270,

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H.E.J. Research institute of Chemistry, International Center for Chemical and Biological

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Sciences, University of Karachi, Karachi 75270, Pakistan.

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Pakistan.

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Corresponding author Farhat Ikram

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(E-mail: [email protected] and [email protected])

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ACCEPTED MANUSCRIPT Abstract Epicatechin capped silver nanoparticles (ECAgNPs) were examined as sensor for Pb2+ in blood and water samples. Several metal ions including Pb2+, Ca2+, Ni2+, Cd2+, Zn2+, Co2+, Hg2+, Cu2+, In3+, K+, Li+, Na+, NH4+, Cu+, Sn2+, Ba2+ and Bi3+ were screened. UV-visible spectra showed

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that only Pb2+enhanced absorption intensity (hyperchromic shift) at 412 nm whereas no other

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tested metal indicated any substantial change in the absorption spectra of ECAgNPs.

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Characterization of ECAgNPs-Pb2+ complex through AFM showed spherical morphology, particle size measured through DLS was found to be 86.06 nm and Job’s plot indicated

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1:1binding stoichiometry of this complex. Whereas, ECAgNPs can selectively detect Pb2+ in a

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concentration range of 1-100 μM with the detection limit down to 1.52μM. Variation in pH from 1-12 showed that ECAgNPs-Pb2+ complex is highly stable in basic medium. The results

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concluded on the basis of above mentioned techniques signifies remarkable selectivity of

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ECAgNPs for Pb2+ over an extensive range of metal ions. Therefore, the synthesized ECAgNPs

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can successfully detect Pb2+ in tap water and blood samples.

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detection; Pb2+.

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Keywords: Epicatechin; silver nanoparticles; nanosensor; water treatment; metal

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ACCEPTED MANUSCRIPT 1. Introduction Nowadays, worldwide industrial progress has indeed brought prosperity in quality of life but it also resulted in increased anthropogenic activities causing release of wide spectrum of toxins in air, soil and water systems. [1] Among these toxins, heavy metals pose most serious threat due to

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their persistent and non-biodegradable nature. Their accumulation in animal tissues, vegetables

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and fruits cause propagation through food chain. [2],[3] Lead is one of those element that enter in

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environment due to varied industrial processes such as mining, smelting, metal plating, storage battery manufacturing and fossil fuel burning etc. [4],[5] Furthermore, it mainly enters into

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human bodies through lead contaminated drinking water or food having lead leaching through

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corroded plumbing systems. [6] Lead is highly toxic even at low levels and it may cause anemia, memory problems, stomach cancer, kidney damage, nerve damages and lung cancer. That’s why

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lead concentration must be controlled in drinking water and for that purpose nanosensors are

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getting more attention these days. [7]-[13]

If concentration of metal ions in environmental samples is low then their detection becomes a

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difficult task. Sometimes similar physical and chemical properties of some metals make it more

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challenging to detect and determine their concentration levels. In environmental samples lead has been detected through different methods and these include; atomic absorption spectrometry

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(AAS) [14], inductively coupled plasma-mass spectrometry (ICP-MS) [15], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [16], quantum dots for spectrophotometric detection. [17] Electroanalytical techniques such as stripping voltammetric methods [18], Ionimprinted polymers (IIPs) based electrochemical sensors [19], surface modified electrodes [20] can also determine trace level of metal ions but interference by other metal ions affect its selectivity. All above mentioned methods require expensive equipment and laborious sample pretreatment steps. 3

ACCEPTED MANUSCRIPT Nowadays in environmental samples heavy metal concentrations can be determined through nanomaterials because such systems are cheap and need less energy input. Therefore, nanosensors found their application as sensors for metals, [21]-[25] due to their remarkable spectrophotometric properties, small size, biocompatibility and low toxicity [26] and stabilizing

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agents such as citrates, polymers, polysaccharides and proteins are usually used to enhance their

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properties. [27] Polyphenols are also well known stabilizing agents. [28] Polyphenolic compounds

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are preferred to use for complexation due to the coordination of their carbonyl and hydroxyl groups with metal ions. Epicatechin is one of the most abundant and widely spread flavonoids; it is a

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polyphenol derivative and found extensively in food products and plants. Since epicatechin is a

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polyphenol, we expected that it will stabilize the nanoparticles.

In this research we reported an economical, sensitive and selective method for Pb2+ detection in

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environmental samples. When ECAgNPs were added separately in each water sample containing

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various metals, increase in absorbance intensity was observed only with Pb2+ which is due to the interaction of ECAgNPs with Pb2+ and this interaction depends upon molecular configuration, so

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it favors Pb2+ binding with the synthesized nanosensor more than several other metals.

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2. Materials and Methods

2.1 Materials and instrumentations

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All the analytical grade chemicals were purchased from Sigma Aldrich and all the spectroscopic grade solvents were procured from RiedeldeHaen and used as received. In deionized water metal solutions of similar concentration (100 µM) were prepared and epicatechin

was

dissolved

in

2:8

(methanol:water,

v/v)

ratio.

Thermo

Scientific

Evolution 300 Spectrophotometer was used to record UV-visible spectra. For AFM analysis, sample solution (100 mg/ml) was vortexed for one minute followed by 30 minute’s sonication.

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ACCEPTED MANUSCRIPT 10 μL of this solution was deposited on a freshly cleaved mica surface to observe images using AFM 5500 (Agilant, USA) in tapping mode. For FT-IR spectral analysis Bruker-Victor-22 was used. For pH measurements a pH meter model 510 (Oakton, Eutech) having a reference Ag/AgCl electrode and a glass working electrode was used. Through dynamic light scattering

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(DLS) particle size was measured using Malvern Zetasizer Nano-ZS 90 (Malvern, UK). In the

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analysis following assumptions were made: the viscosity and refractive index of solution was

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assumed to be same as water (η = 0.888 mPa.s, n = 1.33); disposable polystyrene cuvettes were

2.2 Synthesis and Characterization of ECAgNPs

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used at 25°C temperature.

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Detailed description of synthesis and characterization have been given in our recent publication. [29] A brief summary is as follows:

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Epicatechin solution was added drop wise to silver (I) nitrate followed by addition of

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triethylamine and the resulting yellow suspension was centrifuged to obtain ECAgNPs in the form of precipitates. Synthesized ECAgNPs were characterized through AFM, FTIR, SEM, DLS

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and UV-visible spectroscopy.

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2.3 ECAgNPs as Pb2+ sensor

Different metal (Pb2+, Ca2+, Ni2+, Cd2+, Zn2+, Co2+, Hg2+, Cu2+, In3+, K+, Li+, Na+, NH4+,

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Cu+, Sn2+, Ba2+ and Bi3+) solutions of similar concentration (100 μM) were prepared. Metal solution and ECAgNPs were mixed (1:1, v/v ratio) to record UV-visible spectra. Interfering effect of other metals on Pb2+ sensing was also studied by observing UVvisible spectra of ECAgNPs-Pb2+complex with added several metals. The presence of Pb2+ was checked in tap water (collected from University of Karachi, Pakistan) using ECAgNPs by preparing two different Pb2+ solutions of same concentrations (i.e. 100 μM)

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ACCEPTED MANUSCRIPT in deionized water and in tap water. After that, ECAgNPs were mixed in both deionized and tap water solutions in 1:1 (v/v) ratio and absorption spectra were recorded. In a heparinized tube blood sample was collected and centrifuged at 4000 rpm for five minutes to separate plasma from blood. After that two stock solutions in deionized water were

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prepared by adding plasma (1 mL) and ECAgNPs (1 mL) in both the solutions, whereas

3. Results and discussion

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of ECAgNPs in presence and in absence of Pb2+.

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Pb2+ (1 mL) was added only in one solution to check the alterations in absorption spectra

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Epicatechin and its silver coated nanoparticles (ECAgNPs) were characterized through UV-

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visible spectroscopy, FTIR, AFM, DLS and SEM. UV-visible spectra of epicatechin and ECAgNPs showed max at 280 nm and 412 nm (Figure 1a). AFM indicated spherical

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nanoparticles with maximum particle size in the range of 12-24 nm. FTIR spectral analysis of

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ECAgNPs presented absorbance band shifting from 3431 cm−1 to 3413 cm−1 and SEM

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micrographs exhibited spongy nature of ECAgNPs. Particle size of ECAgNPs observed through DLS histogram was 74.38 nm. For detailed discussion, our previous publication may be referred.

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[29]

3.1 Application of ECAgNPs in Pb2+ sensing

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Metal nanoparticles due to their enormous surface to volume ratio possess most distinctive imperative properties. Silver nanoparticles in particular reveal extraordinary characteristics correlated to noble metals, all of these properties as well as the absorption intensity and max of localized surface plasmon resonance (LSPR) band depends upon distribution, size, aggregation and shape of nanoparticles. [30] Mixing of epicatechin with Pb2+ resulted quenching in absorbance intensity, whereas ECAgNPs induced enhancement in absorbance intensity (Figure

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ACCEPTED MANUSCRIPT 1a). Although both epicatechin and its silver nanoparticles showed interaction with Pb2+ but nanoparticle formation shifted its max from UV to visible region and epicatechin alone due to its low absorption intensity cannot be used as sensor, that’s why ECAgNPs acted as colorimetric sensors. [31][32]

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The binding selectivity of synthesized nanosensor was evaluated against various metal ions such

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as Pb2+, Ca2+, Ni2+, Cd2+, Zn2+, Co2+, Hg2+, Cu2+, In3+, K+, Li+, Na+, NH4+, Cu+, Sn2+, Ba2+ and

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Bi3+through UV-visible spectroscopy. Pb2+ was the only metal ion that showed the hyperchromic shift upon binding with epicatechin based silver nanoparticles as shown in Figure 1b. It may

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suggest that ECAgNPs have competent binding sites only for Pb2+. That’s why ECAgNPs

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interaction with Pb2+ is more favourable than other added metals. 3.2 Selectivity studies of ECAgNPs as Pb2+ sensor

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Selectivity of ECAgNPsfor Pb2+ was investigated by adding Pb2+ solution (100 μM) to

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ECAgNPs (100 μM) with other metal ions (100 μM) such as,Ca2+, Ni2+, Cd2+, Zn2+, Co2+,

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Hg2+, Cu2+, In3+, K+, Li+, Na+, NH4+, Cu+, Sn2+, Ba2+ and Bi3+. Results of this study are presented in Figure 1c and Figure 1d, which illustrates that competing metals did not

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indicate any noticeable effect in the ECAgNPs-Pb2+ interaction. 3.3 Analytical applications in real water and plasma samples

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Water analysis from household plumbing fittings (connecting pipework and taps) showed significant amount of lead ranges from 5.21x102- 6.96x103 μM. For instance, one of the major source of lead in water are screws which contained 3.05x104μM lead. In Karachi surface and ground water analysis revealed lead amount greater than 0.72 μM. [33] World health organization defined maximum acceptable concentration of lead in drinking water that is, 0.050.07 μM. [34]

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ACCEPTED MANUSCRIPT Due to the alarmingly high concentration of Pb2+ in drinking water, sensing ability of ECAgNPs for Pb2+was examined by increasing concentration of Pb2+ from 1 to 100 μM, whereas concentration of ECAgNPs was kept constant. The absorbance intensity of ECAgNPs enhanced gradually as the concentration of Pb2+ increased. (Figure 2a) Limit of detection and limit of

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quantification calculated through standard deviation of blank and slope of regression line were

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found to be 1.52μM and 7.53μM respectively. [35] Results obtained through UV-visible spectra

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indicated strong interaction between ECAgNPs and Pb2+, that’s why this method is useful for the detection of high concentration of Pb2+ in water.

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For the evaluation of ECAgNPs as nanosensor, two different Pb2+ solutions were prepared in tap

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water (collected from University of Karachi, Pakistan) and in deionized water. Absorption spectra of ECAgNPs with Pb2+ in tap water (Figure 2b) showed enhancement in absorption

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intensity and it was not affected by electrolytes present in tap water of Karachi

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Furthermore spectral analysis of Pb2+ through ECAgNPs in plasma also showed the hyperchromic shift in absorbance intensity which reveals that ECAgNPs can successfully sense

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Pb2+ in plasma (Figure 2c) and components of plasma do not alter the sensing ability of

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ECAgNPs. These observations further confirmed that ECAgNPs acted as a selective sensor for the detection of Pb2+ in water and blood sample.

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3.4 Effect of pH on the stability of ECAgNPs and Pb2+ complex An optimized pH is essential for spectroscopic sensing because of its vital role in complex formation and the host guest complex is usually pH dependent. Figure 3a shows stability of ECAgNPs-Pb2+ complex by varying pH in the range of 1-14. The absorption intensity of ECAgNPs-Pb2+ complex increased with increment in pH from 10–12, which is due to the enhancement of nucleophilic character of donor atom i.e. oxygen atom and it facilitate hydrogen

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ACCEPTED MANUSCRIPT bonding. Absorption intensity of ECAgNPs-Pb2+ complex decreased for pH range of 1 to 9 which may be due to the deprotonation of ECAgNPs. The pKa values are pKa1 = 8.91, pKa2 = 9.93 and pKa3 =11.76, for epicatechin. These values shows that it is a weak acid and partially dissociated in medium. As we increased pH of this complex (greater than 7), some of the free

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hydroxyl groups in medium can combine with Pb2+ and produce white precipitates of Pb(OH)2,

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however small concentration of lead ions didn’t lead to any precipitation. [36] These results

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suggest that ECAgNPs can efficiently sense Pb2+ in the pH range of 1–12. [37] 3.5 Particle size determination of ECAgNPs-Pb2+ complex

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The size and morphology of ECAgNPs-Pb2+ complex was recognized through AFM and analysis

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showed presence of spherical ECAgNPs-Pb2+ complex. The images corresponding to ECAgNPsPb2+ complex showed particle size distribution ranges from 4-47 nm with maximum size in the

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range of 28-33 nm (Figure 3b). ECAgNPs may bind Pb2+ through hydroxyl group (-OH) of

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epicatechin and increment in size of ECAgNPs-Pb2+ complex as compared to ECAgNPs is an indication of complex formation.

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Particle size analysis through dynamic light scattering showed increment in size from 74.38 to

(Figure 3c).

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3.6 Job’s plot

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86.06 nm which is due to the interaction of Pb2+ with ECAgNPs in ECAgNPs-Pb2+ complex

Job's plot was used to determine binding stoichiometry between ECAgNPs and Pb2+in ECAgNPs-Pb2+complex. The total concentration of both Pb2+ and ECAgNPs was kept constant (100 μM) and mole fraction of Pb2+ was varied from 0.1-1. In Job’s plot absorption intensity was plotted against mole fraction which revealed 1:1 binding stoichiometry between ECAgNPs and Pb2+ (Figure 4a).

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ACCEPTED MANUSCRIPT 3.7 Determination of binding constant (kb) Through Benesi-Hildebrand formula binding constant between ECAgNPs and Pb2+was estimated with an assumption that only single type of interaction exists between ECAgNPs and Pb2+ [38]. 𝟏

𝟏

𝟏

= ∆𝛆[𝐄𝐂𝐀𝐠𝐍𝐏𝐬] + ∆𝛆[𝐄𝐂𝐀𝐠𝐍𝐏𝐬]𝐤 × [𝐏𝐛𝟐+]

(1)

𝐛

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𝟏 ∆𝐀

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Where change in absorbance of ECAgNPs in presence and in absence of Pb2+ is ΔA and

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change in absorption coefficient is Δε, [ECAgNPs] and [Pb2+] are the concentration of

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epicatechin cappped silver nanoparticles and lead ions. The plot of 1/[Pb2+] versus 1/ΔA was linear and shows good correlation (R2= 0.999) which confirms 1:1 binding ratio between

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Pb2+ and ECAgNPs found through Job’s plot (Figure 4b).

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The binding constant (Kb) from the ratio of intercept/slope was found to be 4.54 x 102 M-1. ∆Go for Pb2+and ECAgNPs interaction was calculated using following equation (2).

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∆𝐆𝐨 = −𝐑𝐓𝐥𝐧𝐤 𝐛

(2)

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Where T is temperature and R is ideal gas constant, for Pb2+ binding ∆Go was found to be

favourable. Conclusion

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4.

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-15.16 KJ mol-1 which suggests that ECAgNPs-Pb2+ complex formation is energetically

In conclusion, ECAgNPs can selectively detect Pb 2+ in the presence of other interfering metal ions. Based on its low detection limit, easy preparation, excellent selectivity and low cost, ECAgNPs can be recommended as an effective sensor for routine monitoring of Pb2+ levels in environmental samples. This study could also potentially lead to many more sensors being designed by this approach using flavonoids in their core skeleton. References 10

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inclusion complexes of β-cyclodextrin with vitamin molecules,” Sci. Rep., vol. 6, pp.

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35764, Oct. 2016.

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Environ. Saf., vol. 129, pp. 103–108, Jul. 2016.

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ACCEPTED MANUSCRIPT List of Figures Figure 1(a). UV-Visible spectra of epicatechin, Pb2+, ECAgNPs and epicatechin-Pb2+ complex, max was observed at 280 nm and 412 nm for epicatechin and ECAgNPs. Figure 1 (b). Application of ECAgNPs as nanosensor for various metal ions, black bar indicates absorption intensity of ECAgNPs and red bar shows absorption intensity of ECAgNPs with other added metals: 2 = Pb2+ , 3=Ca2+, 4=Ni2+, 5=Cd2+, 6=Zn2+, 7=Co2+, 8=Hg2+, 9=Cu2+, 10=In3+, 11=K+, 12=Li+, 13=Na+, 14=NH4+, 15=Cu+, 16=Sn2+, 17=Ba2+ and 18=Bi3+ at 412 nm.

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Figure 1(c). Sensing ability of ECAgNPs for Pb2+ in presence and in absence of various metal ions: 3=Ca2+, 4=Ni2+, 5=Cd2+, 6=Zn2+, 7=Co2+, 8=Hg2+, 9=Cu2+, 10=In3+, 11=K+, 12=Li+, 13=Na+, 14=NH4+, 15=Cu+, 16=Sn2+, 17=Ba2+ and 18=Bi3+ was observed.

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Figure 1(d). Effect of interfering metal ions on epicatechin nanosensor for Pb2+ detection, where black bar represents absorption intensity of ECAgNPs, red bar represents absorption intensity of ECAgNPs+Pb2+ and blue bar shows absorption intensity of ECAgNPs+Pb2+ with other added metals: 3=Ca2+, 4=Ni2+, 5=Cd2+, 6=Zn2+, 7=Co2+, 8=Hg2+, 9=Cu2+, 10=In3+, 11=K+, 12=Li+, 13=Na+, 14=NH4+, 15=Cu+, 16=Sn2+, 17=Ba2+ and 18=Bi3+ at 412 nm.

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Figure 2(a). Effect of Pb2+ on the absorption intensity of ECAgNPs with different concentrations of Pb2+ (1-100 μM) at 412 nm. Figure 2(b). Application of ECAgNPs for Pb2+ detection in tap water sample. Figure 2(c). A UV-visible spectra shows Pb2+ detection in human blood plasma. Figure 3(a). Effect of pH on the stability of ECAgNPs-Pb2+ complex.

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Figure 3(b). AFM images of ECAgNPs-Pb2+ complex displayed spherical particles. Size distribution of ECAgNPs-Pb2+ complex showing maximum particle size distribution in the range of 28-33 nm.

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Figure 3(c). The percent intensity size distribution of ECAgNPs-Pb2+ complex showing particle size of 86.06 nm.

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Figure 4(a). Job’s plot for ECAgNPs-Pb2+ complexes. The total concentration of Pb2+ and ECAgNPs was 100 μM. Job’s plot showed 1:1 mole ratio of ECAgNPs-Pb2+ complex Figure 4(b). Benesi–Hildebrand plot of ECAgNPs-Pb2+ complex based on 1:1 binding stoichiometry. The absorbance intensities were recorded at 412 nm.

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ACCEPTED MANUSCRIPT Highlights

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Epicatechin coated silver nanoparticles acted as a remarkable sensor for Pb2+ detection in water and plasma samples. Presence of various metal ions didn’t interfere the sensing ability of epicatechin coated

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Epicatechin coated silver nanoparticles complex with Pb2+ is stable in the pH range of 1-

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silver nanoparticles for Pb2+.

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Figure 1

Figure 2

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Figure 4