Cefuroxime derived copper nanoparticles and their application as a colorimetric sensor for trace level detection of picric acid

Abstract

Picric acid (PA) is an explosive material often used in explosive tools, destructive weapons and nuclear technology. In this work we demonstrate a simple, novel and efficient wet chemical protocol for fabricating copper nanoparticles using cefuroxime drug as a protecting agent. Cefuroxime functionalized copper nanoparticles (Cefu-CuNPs) were initially confirmed and characterized via UV-Visible (UV-Vis) spectroscopy. Surface interaction between cefuroxime and copper nanoparticles was investigated by Fourier transform infrared (FTIR) spectroscopy while their shapes and size were observed via Atomic Force Microscopy (AFM) where an average size of 30.3 ± 2 nm was recorded. As-synthesized Cefu-CuNPs were used as a colorimetric sensor for detection of picric acid. Cefu-CuNPs demonstrated highly sensitive and selective colorimetric detection of picric acid in the linear range of 1.0–20 μM based on increase in intensity with an R² value of 0.995 determined by UV-Vis spectroscopy measurements. The resulting sensor is highly economical, simple compared to other sensors and sensitive to detect picric acid with a detection limit as low as 38 nM. The sensor can be successfully employed for picric acid detection in real water samples.

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1. Introduction

Terror attacks in several countries have attracted much attention from many researchers to discover effective sensors for the detection of explosives. Generally, nitro compounds are used as crucial constituents in many explosives and the most commonly used compound among the NACs is TNT.^1–3 Picric acid (PA) has recently been introduced as a potential substitute due to its great explosive power and sufficient solubility in water. It is commonly used as a reagent in many industries such as leather, dye, rocket fuel manufacture, and pharmaceutical.⁴ PA is also used to manufacture nitro trichloro methane (CCl₃NO₂) or chloropicrin, a strong insecticide having excellent anti-microbial and anti-fungal properties.⁵ There are many problems like liver malfunction, skin and eye irritation, anemia, cancer, and cyanosis associated with ingestion of PA. Therefore, to save the community and environment, selective and sensitive detection of PA is very necessary.^6,7 Several sensors such as polymer based sensors, nanosensors, fluorescence quenching based sensors, quantum dot antibody fluorescence resonance energy transfer electrochemical based sensors and nanomaterial surface energy transfer have been used for the detection of trace explosives.⁸

Metal nanoparticles (NPs) have been utilized for developing several sensors, associated with various spectroscopic techniques such as surface enhanced Raman scattering (SERS)⁹ resonant light scattering (RLS),¹⁰ absorption,¹¹ fluorescence¹² and so on. Among optical sensors, Au and AgNPs based colorimetric methods have been reported to detect metal ions, proteins and other small molecules.^13–17

Metal nanoparticles (NPs) are of high interest due their potential applications in many fields like, electronics, catalysis, optical devices, sensors and biotechnology. Various synthetic strategies have been adopted for preparation of NPs such as laser ablation,¹⁸ microwave irradiation,¹⁹ vacuum vapor deposition²⁰ and thermal reduction²¹ methods. Among these, chemical reduction protocol in aqueous medium is preferred due to the compatible analyses in aqueous environment. Now-a-days, the use of metal nanoparticles (NPs) as colorimetric sensors has been an easier, simpler and economical practice. NPs particularly Cu, silver (Ag) and gold (Au) NPs have attracted much attention as compared to other precious metal NPs due to their mesmerizing and exceptional property of Localized Surface Plasmon Resonance (LSPR).^22,23 When particle sizes attain nanoscale regime, their conduction band and surface electrons oscillate collectively to generate prominent spectral excitation band. Metal NPs like Cu, Ag and Au with dimension in the range of 10–60 nm demonstrate LSPR band around 570 nm, 400 nm and 520 nm, with prominent red, yellow and light red solution respectively^24–28 which have been applied in optical sensing of different analytes. Although, Ag and Au NPs are associated with exceptional optical properties but comparatively their higher cost restricts to develop economical sensors on commercial scale production.²⁹

Unfortunately for plasmonic application, fabrication of pure CuNPs is a challengeable task. CuNPs have tendency to be oxidized in an aqueous system which is the major drawback for using these particles as colorimetric sensors. To overcome such problems various strategies such as use of an inert environment, stabilizers, suitable polymers and non-aqueous solvent system were applied. For example, a group of researchers prepared Cu nanoparticles using polymer resembling NaH₂PO₂ and PVP without providing inert system.³⁰ However, the procedure was thermo stabilized and time consuming. In another reported study, CTAB was employed to stabilize the resulting CuNPs while N₂H₂ was used for providing inert gas.³¹ Zhang et al.³² used gelatin as capping source for synthesizing CuNPs. Since gelatin is not soluble in water at room temperature, so the dispersion of particles in water would have been difficult. Still no convenient method to synthesize Cu nanoparticles in aqueous environment has been reported.

In this research article we report the synthesis of Cu nanoparticles using cefuroxime assisted reduction method in which cefuroxime acts as capping agent. Also, no any procedure is available in the literature on colorimetric detection of picric acid using CuNPs. This paper reports for the first time the colorimetric detection of picric acid in nanomolar level.

2. Experimental

2.1. Chemicals

Analytical grade reagents and chemicals were used in the undertaken studies. CuCl₂ 5H₂O (97%), sodium borohydride (NaBH₄), (80%), were purchased from Acros Organics New Jersey, USA. HCl (37%), NaOH (98%) and various salts including HgSO₄ H₂O, Cd (NO₃)₂, Ni (NO₃)₂ 6H₂O, Co (NO₃)₂ 6H₂O, CrCl₃ 6H₂O and As₂O₃ were obtained from Sigma-Aldrich, whereas cefuroxime, 4-nitroanaline, pentachlorophenol and hydroquinone from Fluka Chemicals, while picric acid was obtained from Scharlau. All the solutions were prepared by dissolving a specific amount (per mg) of each in pure Milli-Q water (preparatory reagent) and all experimental work was performed at room temperature (25 °C).

2.2. Instrumentation

UV-visible spectrometer (Lambda 35 of Perkin-Elmer) was used for recording LSPR bands of Cefu-CuNPs and colorimetric assay measurements were conducted within the UV-Vis range of 200–800 nm. Surface modification of CuNPs with cefuroxime was observed by Fourier transform infrared (FTIR) spectrometer (Nicolet 5700 of Thermo) by making KBr pellets. In order to obtain the sample in solid form the solution was dried in water bath.

Morphological characterization for shape homogeneity and size distribution of formed nanoparticles (Cefu-CuNPs) was performed using AFM (model AFM 5500 Agilent, USA). Similarly, photographs of the Cefu-CuNPs used for visual colorimetric response of picric acid were recorded by using a digital camera.

2.3. Synthesis of copper nanoparticles

The synthesis of Cefu-CuNPs was performed in a capped graduated test tube. In a typical experiment, 0.3 mL of 0.01 M solution of CuCl₂ was diluted with milli Q water up to 8 mL taken in the test tube. Furthermore 0.010 mL of 0.01 M cefuroxime solution and 0.1 mL of 0.1 M NaBH₄ was added and finally diluted to 10 mL mark with deionized water. The reaction was observed and it was seen that the transparent reaction mixture changed to deep brown color after 20 min. The pH of entire mixture was kept neutral in order to reduce the particle size. The change in color confirmed the formation of CuNPs and the final product was used for colorimetric detection.

2.4. Detection of picric acid by Cefu-CuNPs

The colorimetric sensing of aqueous picric acid was experimentally performed at normal temperature. Various volumes 10, 30, 50, 70, 90, 110, 150 and 200 μL of picric acid stock solution having 0.001 M concentration were added to 3 mL of Cefu-CuNPs solution and made up to 10 mL. The solution mixtures were kept for 3–4 min, and then small portions of these solutions were transferred in to 1 cm quartz cell to analyze the spectral response. Absorbance was measured at wavelength range from 200 to 800 nm against blank solution. Variation in color from red to yellow was taken as visual response of the colorimetric system. The photographs of variation in color of solutions were also obtained with a 16 mega pixel camera after 5 min of reaction time.

2.5. Detection of picric acid in real samples

Here the blank samples of various water collected from various localities were first mixed with Cefu-CuNPs solution and then spiked with different concentrations of the picric acid solution in similar way as true in case of standard solution. Triplicate runs were recorded and the data displayed in Table 2.

3. Results and discussion

3.1. UV-Vis spectroscopic analysis of cefuroxime capped copper nanoparticles

Several studies have shown that optical properties of metal nanoparticles depend upon the geometry and size; thus the optical signal of metal nanoparticles (NPs) can be controlled to vary the size and shape of metal nanoparticles.^33,34 Surface Plasmon Resonance (SPR) frequencies of metallic nanoparticles for instance Ag, Au and CuNPs reside within the visible scale of electromagnetic spectrum.^35,36 The verification of formed copper nanoparticles was carried out by using optical spectroscopy as primary tool. For this reason optimization of a range of parameters such as concentration of cefuroxime, sodium borohydride and copper chloride was carried out. The small sized and extremely stable Cefu-CuNPs were obtained by taking suitable volumes of the salt solution. Experimental results are illustrated in Fig. S1(a) (ESI†) and indicate an increase in intensity with small changes in LSPR band wavelength with higher amount of Cu ions (0.1–0.8 mL of 0.01 M). However, with higher volume of Cu ions >0.8 mL, the LSPR band appeared as red shifted. This change in LSPR peak may due to increased nucleation growth with huge Cu(II) ions freely available in solution creating smaller nanoparticles.^37,38 The spectral profile of (0.1–0.6 mL of 0.1 M) reducing agent suggests the reduction in particle size is indicated from the blue-shift in LSPR wavelength from 580 nm to a shorter wavelength of 570 nm (Fig. S1(b)†). UV-Vis spectral profile in Fig. S1(c)† shows that an increase in concentration of capping or protecting agent (cefuroxime) affects the LSPR band shape and a blue-shift slightly from 575 nm to 572 nm is noticed. It is clearly defined previously that the exact position of the LSPR band depends on various factors such as, composition, shape, capping agent, size of particles and aggregation state.³⁹

Time duration study of the formed CuNPs was recorded in order to check the stability of the formed nanoparticles. Results show that there was no change in wavelength and color of the solution. Previously it has been studied that CuNPs do not lose their stability in an inert environment. As a result of oxidation due to dissolved oxygen in an aqueous solution, the intensity of LSPR peak and concentration of synthesized nanoparticles become gradually decreased as per reported study.⁴⁰ In the present study without providing any inert environment, absorption intensity of formed nanoparticles remained unchanged for more than 2 months after formation and no oxidation and precipitation occurred as shown in Fig. 1.

Fig. 1 Time study of Cefu-CuNPs showing stability of the sol.

This indicates that the Cefu-CuNPs are more stable in aqueous solution in the presence of optimal amount of capping agent (cefuroxime) and reducing agent (NaBH₄).

3.2. Fourier transforms infra-red (FT-IR) spectroscopy

FTIR study was carried out in order to investigate the surface interaction of cefuroxime molecules with Cu metal. Due to the presence of several functional groups cefuroxime shows various bands that depend upon the vibrational modes such as stretching and bending.⁴¹ Fig. 2(a) and (b) show the FTIR spectra of pure cefuroxime and Cefu-CuNPs respectively. Various characteristic bands in Fig. 2(a) can be interpreted as: asymmetric and symmetric stretching of (COO⁻¹) at 1626 and 1401 cm⁻¹ respectively, (–NH) bending vibration at 1545 cm⁻¹ and the band of (–NH) stretching at 3365 cm⁻¹. Out of these a peak assigned to (–OH) group of cefuroxime molecule was observed near 3256 cm⁻¹ while peak at 1759 cm⁻¹ confirms the presence of carbonyl group (–C=O) in standard cefuroxime.⁴¹

Fig. 2 FTIR spectra of (a) pure cefuroxime and (b) cefuroxime capped CuNPs.

The FTIR spectrum of Cefu-CuNPs (Fig. 2(b)) elaborates nearly similar vibrational peaks however the peak due to (–NH) as present in case of Fig. 2(a) was shifted towards shorter wave number in this spectrum which confirms cefuroxime is more likely to bind at the surface of CuNPs through amino group which was previously reported by Ramajo et al.⁴² in case of Ag nanoparticles. This interaction is further confirmed while considering the spectrum of Cefu-CuNPs. However it has been seen that if both carboxyl and amino groups are present in a molecule the better chance of interaction of metal nanoparticles occurs with amino group rather than carboxyl group.⁴³ Furthermore variations such as the shift in the vibration frequency of carbonyl (COO⁻¹) stretching from 1626 cm⁻¹ to 1480 cm⁻¹ and band at 1759 cm⁻¹ for carbonyl group and other functional group frequencies after interaction with CuNPs can be discussed regarding to dipole moment change after binding of cefuroxime on the high electron surface of metal as previously reported for detection of Hg(II).⁴⁴

3.3. Colorimetric detection of picric acid

Typically, CuNPs exhibits red color owing to its LSPR peak around 400–600 nm in UV-Visible range of electromagnetic spectrum.⁴¹ The visible spectrum of Cefu-CuNPs is reported in Fig. 3(a). It is clear that LSPR band resides at 566 nm with a narrower band shape, confirming the uniform distribution of particles and crystalline nature within the aqueous environment. When picric acid interacts with Cefu-CuNPs, a colorimetric change from red (Fig. 3(a)) to yellow (Fig. 3(b)) corresponding to change in the spectral profile of Cefu-CuNPs from narrow, blue-shifted band to broader and red-shifted band with increased intensity was observed as illustrated in Fig. 3.

Fig. 3 UV-Visible spectral profile of (a) Cefu-Cu NPs, and (b) interaction of Cefu-CuNPs with picric acid.

The observed changes in spectral profile while interaction with picric acid may be defined on the basis of agglomeration of particles within the aqueous environment.

The visual scenario regarding dimensional changes based on aggregated and non-aggregated form of CuNPs was studied by the AFM analysis. It was clearly observed that without presence of picric acid, the Cefu-CuNPs formed were monodispersed and spherical with an average size of 30.3 ± 2 nm ranging from 15 to 80 nm as shown in Fig. 4(a) and (c). These dimensional changes support the LSPR spectral band of Cefu-CuNPs studied in Fig. 3(a). The present study clearly reflects that the interaction between the drug (cefuroxime) and surface of CuNPs was responsible for the formation of spherical and stable nanoparticles. Amide group of cefuroxime takes part in coordination with CuNPs to prevent aggregation and acts as capping agent as well as stabilizing agent as previously mentioned in case of paracetamol-AgNPs.⁴⁵

Fig. 4 AFM images of (a) Cefu-CuNPs, (b) Cefu-CuNPs + 20 μM picric acid and (c) size distribution histogram of (a) within the range of 15–80 nm.

AFM image in Fig. 4(b) obtained at same time after the interaction of Cefu-CuNPs with picric acid (20 μM) illustrates the formation of heavy aggregates. Furthermore, this AFM image clearly shows that picric acid interacts with capping agent to remove it from the surface of CuNPs in order to get these particles aggregated.

Thus, such aggregation counts for the increase in band intensity of picric acid and change in the peak of Cefu-CuNPs towards shorter wavelength with increased intensity as previously reported in case of AgNPs.⁴⁶ Furthermore the UV-Vis spectrum at 359 ≈ 360 nm has been described to be due to removal of cefuroxime from the surface of CuNPs and the interaction of its amino moiety with PA.⁴⁷ As the capping agent is removed from CuNPs via interaction with PA, hence the later are getting aggregated as evidenced from the Fig. 4(b). Similar result has been illustrated in Fig. S2 (ESI†) when 0.01 M solution of cefuroxime mixed with 0.01 M solution of picric acid and spectrum resulted in peak at 360 nm. In the light of these illustrations and supportive data, we can suggest the following two schemes for the formation of Cefu-CuNPs and appearance of new band at 359 nm which provided the platform for colorimetric sensing of PA (Schemes 1 and 2).

Scheme 1 Proposed mechanism for formation of cefuroxime derived stable CuNPs.

Scheme 2 Proposed mechanism showing removal of cefuroxime from stable CuNPs.

Where NH₂ represents cefuroxime while NH₂–CuNPs represents the cefuroxime coated copper nanoparticles and X represents numerous ions, molecules or particles.

Mechanism 2 displays the role of aggregation based conversion of Cefu-CuNPs after reaction with PA as discussed above.

These mechanisms justify the formation of stable CuNPs via cefuroxime and the development of PA sensor as a result of the crucial role of the formed CuNPs.

We conclude that the increase in LSPR band intensity is a consequence of complexation between picric acid and Cefu-CuNPs in which picric acid forms a complex with removed cefuroxime in the presence of aggregated CuNPs which can produce a surface plasmon resonance. Experiment showed that the increase in the intensity of LSPR band at 360 nm is proportional to the picric acid concentration.⁴⁷ So the present experimental work reports a fast, economic, selective and simple colorimetric method for detection of picric acid in aqueous environment using cefuroxime capped CuNPs as a sensing agent.

3.4. Analytical response of colorimetric assay

Quantitative measurements attributed to colorimetric assay were achieved by monitoring the change in the absorbance (LSPR) band of picric acid and Cefu-CuNPs in the optical range of 400–800 nm upon addition of picric acid (Fig. 5(a)). The spectral band at 360 nm resides within the UV-Visible range of electromagnetic spectrum is due to picric acid where as the peak at 566 nm confirms the presence of CuNPs. Increasing in concentration of picric acid, the peak intensity of picric acid increases with red shift in band of Cefu-CuNPs. The color change upon addition of picric acid has been shown in inset photograph of Fig. 5(a).

Fig. 5 (a) Increase in absorbance with increasing concentration of picric acid (b) linear regression plot between absorbance and picric acid concentrations.

The informative data was earned by regression analysis of absorbance (LSPR) at 360 nm plotted against picric acid concentrations. The method concluded excellent linearity in the range 1–20 μM of picric acid concentrations (Fig. 5(b)). The coefficient of determination (R²) was 0.995, with limit of quantification (LOQ) and limit of detection (LOD) determined to be 126.7 nM and 38 nM respectively. The LOQ and LOD were calculated from ten times the value of standard deviation (σ) of the blank (10 × σ/slope) and three times the standard deviation of blank (3 × σ/slope) respectively.

3.5. Selectivity of sensor

To check the selectivity of Cefu-CuNPs for PA sensing several analytes and metal ions including pentachlorophenol (PCP), 4-nitroanaline (4-NA), hydroquinone (HQ), Cd²⁺, Ni²⁺, Co²⁺, As³⁺, Cr³⁺ and Hg²⁺ at the concentration of 100 μM were added to Cefu-CuNPs sol. These metals and important analytes were added into 5 mL of Cefu-CuNPs solution and similar conditions were applied in case of picric acid. The selectivity of sensor can be checked visually as seen in the inset photograph, Fig. 6(a) upon interaction of freshly prepared Cefu-CuNPs with various environmental analytes including metal ions. Only picric acid solution showed color change from red to yellow, while the effect of the other analytes on the color and spectral band of Cefu-CuNPs solution was negligible (Fig. 6(a)), which confirms that the newly developed sensor is highly selective towards picric acid.

Fig. 6 (a) UV-Visible profile of Cefu-CuNPs showing selectivity towards picric acid (b) bar diagram indicating the change in absorbance values for several environmental analytes and cations.

The bar diagrams in Fig. 6(b) represents the extent of interference of various ions and analytes, which clearly shows the selectivity of reported sensor as exemplary.

3.6. Figures of merits for the developed sensor

The developed sensor was compared with other sensors by taking certain properties into account. Table 1 lists the limit of detection, material used and method of operation.

Table 1 Comparative estimation for picric via different methods^a

Method	Material used	Detection limits	Reference
a CTAB-MIONPs, cetyltrimethylammonium bromide-magnetic iron oxide nanoparticles.
Fluorescence	Triphenylamine based compound 1	400 ppb ≈ 1.75 μM	48
Luminescence	ZnO quantum dots	2.86 μM	49
Electrochemical	Reduced graphene oxide	0.54 μM	50
Flow injection electrochemical	Copper electrode	6.0 μM	51
UV-Vis spectrometry	CTAB-MIONPs	31 nM	52
UV-Vis spectrometry	Cefu-CuNPs	38 nM	Present work

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It is evident from the table that most of the sensors reported for PA detection used are associated with complicated synthetic protocols or they use expensive chemicals. In addition they are less sensitive as compared to our developed sensor. We can extend it to naked eye colorimetric detection of PA where there is no need of any instrument. In this list one of the UV-Vis spectrometric based sensor has a better LDL value however the use of several chemicals and complicated synthesis of magnetic iron oxide nanoparticles make it a little bit expensive.

3.7. Application of Cefu-CuNPs for detection of PA in real water samples

In this context, 3 real samples collected from various water resources were analyzed and the data presented in Table 2.

Table 2 Detection of PA in real water samples via developed sensor using recovery method^a

Source of water sample	PA added (μM)	PA recovered (μM)	Recovery (%)
a ±, standard deviation of 3 replicates.
Gajarwa lake	4.0	4.105 ± 0.01	102.6
River Indus	8.0	8.15 ± 0.10	101.9
Keenjhar lake	12.0	11.7 ± 0.20	97.7

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According to the results regarding PA estimation in real samples, the recovery of PA lies in the range of 97.7 to 102.6 within the working linear range of the developed sensor. This verifies the application of our developed sensor in any water matrix.

4. Conclusion

It is concluded that the present work focused on a new strategy to look for a greener, cheaper and facile way of producing highly stable CuNPs using a newer capping agent which easily stabilized these particles up to two month with no danger of oxidation. This synthetic strategy was highly economical, simpler, efficient and less time consuming. Further these stable Cefu-CuNPs were applied as highly sensitive, selective and economical colorimetric sensor for detection of picric acid at nanomolar level. The best merit of the study lies in the fact that highly stable Cefu-CuNPs sol was prepared without the need of any inert environment. This study could thus be extended to use cefuroxime as stabilizing agent for other particles like nickel, cobalt, zinc and so on for similar or other purpose. The linear working range of the sensor works well below and above the permissible limit for PA in water.

Acknowledgements

We acknowledge the Higher Education Commission of Pakistan for financial assistance. We further cordially appreciate and highly thank the Deanship, Scientific Research group of King Saud University for fund provision to this work via their project (PRG-1437-30).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08571g

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