An organic–inorganic nanohybrid of a calix[4]arene based chromogenic chemosensor for simultaneous estimation of ADP and NADH

Abstract

The versatility in the environmental and biological applications of nanohybrids encouraged us to prepare a novel chemosensor based on an organic–inorganic nanohybrid (H1) employing receptor 1 (R1), which was synthesized via the Schiff’s base condensation reaction of a calix[4]arene derivative and an aliphatic amine. Techniques such as DLS and TEM were employed for the characterization of organic nanoparticles (N1) and H1. Further, sensor properties of H1 were explored towards various biologically important molecules in aqueous media using UV-visible spectroscopy. The proposed sensor responded effectively for the selective and simultaneous nanomolar determination of adenosine diphosphate (ADP) and reduced nicotiniamide adenine dinucleotide (NADH). The response was not affected by the presence of each analyte or any other potentially interfering biomolecule or a high concentration of salt. The proposed sensor was also found to show a stable response in an extensive pH range thus widening its practical applicability. H1 was able to detect a minimum concentration (detection limit) of 6.11 × 10⁻⁹ M of ADP and 4.87 × 10⁻⁹ M of NADH. The prepared hybrid was subjected to real sample analysis for the determination of ADP and NADH in samples prepared artificially by adding known concentrations of NADH and ADP in solution and also in a mixture of both.

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Introduction

In recent years, sensors for the quantitative recognition of various biomolecules (amino acids, DNA etc.) increasingly attract the interest of researchers due to their imperative physiological functions and numerous applications in medical diagnostics, anti-bioterrorism and food safety.¹ A standard amount of certain biomolecules is essential for organisms, while an amount that is too high or too low can lead to certain diseases. For example, adenosine diphosphate (ADP) in the form of ADP-ribose is known to play an important role in different cellular processes such as extracellular cell signaling, DNA repair,² gene regulation and apoptosis.³ The function of ADP is imperative in normal hemostasis and thrombosis, and it has a crucial role to play in enzymes functioning as bacterial toxins and metabolic regulators.⁴ Nicotinamide adenine dinucleotide (NAD) plays a part in the regulation of energy metabolism, DNA repair and transcription,⁵ and in maintaining cellular redox homeostasis. The redox couple NAD⁺/NADH represents a very important system, being a substrate for a number of dehydrogenase, hydroxylases and oxidoreductase enzymes in natural biological systems.⁶ An aberrant level of various biomolecules are known to cause many diseases such as hypoglycemia, hypoxia, ischemia, Parkinson’s disease,⁷ renal diabetes, cystic fibrosis and diabetes.⁸ A change in the level of some biomolecules such as NAD/NADH, adenine, thymine and guanine is closely related to many diseases like cancer, AIDS, epilepsy and lupus erythematosus.^9–11 Therefore, it is necessary to develop highly sensitive, prompt, dependable and expedient methods for the detection of biomolecules for various biochemical studies and clinical diagnoses. Detection of these substances requires complicated separation and purification steps, as well as elaborate sample preparation, large costly instruments and skilled labor.¹² A number of artificially prepared receptors employing various organic units such as benzimidazole, urea, thio-urea, and calixarene moieties have already been reported in the literature.^13,14 Within this context, the development of receptors for multi-analyte detection in real samples is still a challenge for researchers. A few chromogenic and fluorescent sensors for the detection of more than one analyte simultaneously have been reported in literature. Such receptors are designed and synthesized either by inserting multi-chromogenic moieties into a single receptor which necessitates tedious synthesis steps or by employing several detection techniques such as UV-visible, electrochemical and fluorescence detection methods.^15,16 Lately, a shift from designing and fabricating a selective receptor to a differential receptor has facilitated the recognition of more than one analyte simultaneously, using one chromophore. Various colorimetric detection techniques employing nanomaterials (nanoparticles, nanorods, quantum dots) have been used in order to get quick, precise, and cost-effective responses in samples of biological and environmental origin these days.¹⁷ Nanomaterials, particularly hybrid nanostructures, are massively employed these days to meet high-end requirements such as inexpensiveness, efficiency and having high sensitivity and selectivity towards particular ions or molecules. Metallic nanoparticles, particularly gold nanoparticles (AuNPs), have been one of the most explored nanostructures lately, owing to their high biocompatibility, conductivity, fluorescent quenching and excellent plasmonic coupling. They exhibit distinctive features such as localized surface plasmon resonance (LSPR)¹⁸ and very high molar extinction coefficients. As per the Beer–Lambert law, these nanoparticles show the possibility of reaching much lower limits of detection (LOD). AuNPs are employed these days for the detection of various biomolecules owing to their comparable sizes. Quantum dots are another class of nanomaterial that emit fluorescence with a high quantum yield and possess quantum confined charge carriers and remarkable electro-chemiluminescent properties.¹⁹ These are known to conjugate with biomolecules (proteins and DNA) with high sensitivity. Organic nanoparticles (ONPs) are a recently explored class of soft matter which possess applications in sensitive and selective chemo-sensing. So, herein, we develop a simple, prompt and responsive colorimetric chemosensor for the estimation of ADP and NADH simultaneously by employing a calix[4]arene derivative and using the interparticle plasmon coupling induced by the aggregation of AuNPs due to binding with a particular analyte.^20,21 Receptor 1, based on the calix[4]arene molecule substituted at the 1,3-alternate positions on the lower rim, is designed in such a manner so as to devise a molecule having tunable cavity size. Such a molecule is bound to have two different cavities competent in accommodating two analytes simultaneously. Because of the presence of the podand arms at the lower rim, the cavity of the calix[4]arene molecule becomes more rigid and is therefore able to selectively and promptly bind a particular analyte of biological or environmental importance. Also, the substituents at the lower rim bear various binding sites which proficiently bind a specific analyte. We report the development of an organic–inorganic hybrid (H1) based chemosensor using receptor 1 (R1) for the simultaneous detection of ADP and NADH in an aqueous medium, having no interference from any other potentially interfering biomolecules. R1 was obtained by the Schiff’s base condensation reaction of 3-(dimethylamino)-1-propylamine and 1,3-disubstituted p-tert butyl calix[4]arene.

Results and discussion

Synthesis and characterization of sensor

The synthesis of R1 was carried out by reacting 3-(dimethylamino)-1-propylamine with the calix[4]arene-based dipodal aldehyde 1 in acetonitrile at room temperature using the previously reported literature method²² (Scheme S1 [ESI†]), which in turn was synthesized by reacting p-tert-butyl calix[4]arene and 2-(2-bromoethoxy)benzaldehyde in acetonitrile as depicted in Scheme S1 (ESI†). The compound R1 (Fig. 1) that formed in 70% yield was white in color and was characterized using elemental analysis.

Fig. 1 Structure of receptor 1.

Fabrication of organic nanoparticles (N1) of receptor 1

Organic nanoparticles (ONPs) are fine suspensions of solid particles, composed of an organic compound in an aqueous medium and ranging in diameter from 10 nm to 1 μm. Because of their optical properties²³ and ability to generate intense electromagnetic fields, these particles have great relevance in various fields such as conducting materials, sensors,²⁴ medicine,²⁵ biotechnology, catalysis, diagnostics²⁶ and photo-thermal therapeutics. Of all the procedures known in the literature for the fabrication of ONPs, the single step re-precipitation technique is usually employed because it is economical and can be easily reproduced. Keeping that in mind, organic nanoparticles (N1) of R1 were obtained by employing the single-step re-precipitation method. For this, 1 mL (0.6 μM) of the compound dissolved in pure tetrahydrofuran (THF) was injected manually into deionised water (100 mL) in a 250 mL beaker, using a micro syringe at a constant rate. The solution was then sonicated for at least half an hour. The change in the photophysical profile of R1 was studied using UV-visible and fluorescence spectroscopy (Fig. 2a and b).

Fig. 2 (a) UV-visible, and (b) fluorescence spectra of ligand in THF and ONPs in water showing noticeable enhancement in peak intensity as ONPs are fabricated.

The organic ligand dissolved in pure THF shows low absorption peaks in the UV-visible spectra. As the amount of water was increased, ONP formation occurred and the solution became slightly cloudy, indicating the uniform distribution of nanoparticles in aqueous medium. The p-tert-butyl calix[4]arene molecule in CHCl₃ is known to show appreciable absorption bands at 230 and 280 nm due to n–π* transitions and these absorption bands are expected to shift towards longer wavelengths as the polarity of the solvent is increased.²⁷ The UV-visible profile of N1 shows peaks at 286 nm and 317 nm attributed to n–π* transitions, while the emission profile shows peaks at 303 nm and 372 nm. Both the spectral profiles show marked enhancement in the peak intensity which can now be used to estimate the changes in the photophysical profile of R1.

The analysis of the N1 particle size was confirmed by TEM (Transmission Electron Microscopy) which showed the development of nanoparticles, spherical in shape having a size of 12 nm (Fig. 5a). The concentration of R1 for the fabrication of ONPs was required to be optimized by varying the injected amount of R1 in 100 mL of distilled water, in order to achieve more intense peaks both in UV-visible and fluorescence spectra. It is quite evidently perceived from the results (Fig. 3a and b), using both UV-visible spectroscopy and fluorescence spectroscopy, that as the amount of R1 was increased from 0.2 μM to 0.6 μM, the intensity of the peaks increased. Upon further increasing the amount of R1, the intensity of the peaks decreased considerably which is ascribed to an increase in particle size leading to agglomeration. The same has been confirmed using DLS studies (Fig. S1†). In order to explore the effect of the amount of water on the photophysical profile of R1, various compositions of water : THF were examined using UV-visible and fluorimetric profiles of this compound (Fig. 4a and b).

Fig. 3 (a) UV-visible spectra and (b) fluorescence spectra of N1 with increasing concentrations of R1.

Fig. 4 Effect of various compositions of the solvent (water : THF) on the (a) UV-visible absorption and (b) fluorescence spectra of N1.

It is quite evident that with an increase in the water concentration, the intensities of the peaks increased considerably and the best peaks are obtained with a composition of 99 : 1, water : THF, therefore providing us with the best suited media for the formation of optimum sized ONPs. The optimum sized ONPs were confirmed using TEM analysis and DLS (Dynamic Light Scattering) and had a size of 12 nm (Fig. 5a and b).

Fig. 5 (a) TEM images of the ONPs in water (N1), and (b) DLS histogram of N1.

Fabrication of the organic–inorganic hybrid (H1)

Many different techniques have been developed to generate organic–inorganic hybrids (H1) using the reduction of gold as gold nanoparticles (AuNPs) on the surface of organic nanoparticles.^26,28–30 For this, stock solutions of HAuCl₄ (1.0 × 10⁻³ M) and ascorbic acid (1.0 × 10⁻³ M) were made using de-ionized water, and subsequent dilutions (100 times) were made from the stock solutions. These dilutions were then tried in different ratios along with the previously prepared N1 (0.1 μM) to obtain H1. It was deduced that HAuCl₄, N1 and ascorbic acid when mixed in the ratio of 9 : 1 : 9 leads to the formation of H1. Formation of H1 from N1 happened with a dramatic change in color from colourless to pink (Fig. S2†). Both the UV-visible and fluorescence spectra (Fig. 6a and b) confirmed the formation of H1, as a clear surface plasma resonance (SPR) band appeared in the visible region at around 528 nm due to the reduction of Au(III) to Au(0) over the organic ligand. In addition, the emission spectra demonstrated quenched emission bands of N1 at 303 nm and 372 nm, attributed to the deactivation of the surface of N1, by the immobilization of AuNPs over its surface. This was further confirmed using TEM (Transmission Electron Microscopy) which showed the appearance of black spots on the surface of N1 with a size of 18–22 nm and also confirmed by DLS studies (Fig. S3†).

Fig. 6 Comparison of (a) UV-visible, and (b) fluorescence spectra of the inorganic organic hybrid nanoparticles (H1) and organic nanoparticles (N1).

It is quite evident from Fig. 6 that the absorption band has clearly shifted from the UV region to the visible region, which clearly justified our decision to prepare inorganic–organic nanohybrids from organic nanoparticles for sensor applications.

Recognition studies

Binding of biomolecules with organic–inorganic hybrids (H1)

In order to study the binding ability of the developed R1 in the form of H1 towards various biomolecules in an aqueous medium, a solution of H1 was taken in a volumetric flask (5 mL) and was subjected to 100 μL (5 μM) solutions of different biomolecules (NAD, NADH, NADP, AMP, ADP, ATP, uracil, adenine, cytosine and guanine) and their UV-visible spectra were noted. It was perceived that upon the addition of biomolecules to the fixed concentration of H1 (6 μM), there were negligible changes in the UV-visible profile with all biomolecules except ADP and NADH (Fig. 7).

Fig. 7 UV-visible profile of H1 in the presence of various biomolecules of 5 μM concentration.

Interaction of NADH with H1 led to a shoulder peak at 648 nm, whereas the addition of ADP caused the appearance of a new peak at 712 nm, along with a decrease in the absorbance peak at 528 nm. Binding of the receptor with anions leads to the stabilisation of the excited state in comparison with the ground state. This leads to a bathochromic shift in the absorption spectrum.³¹ These spectroscopic changes may be assigned to the formation of a charge transfer complex. The observed bathochromic shift, both in the case of ADP and NADH, may be attributed to intermolecular hydrogen bonding between receptor 1 in the form of H1and the biomolecules. To authenticate the binding behaviour of H1 with both ATP and NADH, titrations were performed by adding small aliquots of both separately in a 10 mL solution of H1 (Fig. 8a and b).

Fig. 8 UV-visible spectra of H1 in the presence of increasing concentrations (a) (0–100 nM) of ADP, and (b) (0–80 nM) of NADH (inset (A): calibration plot for ADP, inset (B): calibration plot for NADH).

It is quite evident from the calibration curves that H1 responds linearly for ADP and NADH with a linear dynamic range of 0–100 nM for ADP and 0–80 nM for NADH, having a regression coefficient of 0.9896 and 0.9783, respectively. H1 was able to detect a minimum concentration (detection limit) of 6.11 × 10⁻⁹ M of ADP and 4.87 × 10⁻⁹ M of NADH. The lower detection limit was calculated using the standard IUPAC 3σ method.³²

Further, in order to test it as a sensor for the simultaneous determination of ADP and NADH, a simultaneous binding assay was performed (Fig. S4 & S5†). The experiment was performed by adding one component in excess as the interferent and performing the titration with the second component as the analyte of interest. Calibration curves were plotted for both NADH and ADP, in the presence of each other and compared with the calibration curve of NADH and ADP in the absence of an interferent. It is quite evident from the calibration plots (Fig. S4 & S5†) that the calibration curve remains the same, i.e. neither the slope nor intensity changes upon addition of one component as the interferent. The different behaviours of H1 with NADH and ADP may be ascribed to distinct binding sites in the molecule for both NADH and ADP. For a sensor to have practical applications in real sample analysis besides selectivity, it should be able to work in complex environments, such as in the presence of various potential interferents and varying pH and salt concentration. In order to achieve this, interference studies were carried out and UV-vis spectra were recorded. In 10 mL volumetric flasks, solutions of H1 containing 0.1 μM of each of ADP and NADH were subjected to various other biomolecules (NADP, NAD, AMP, ATP, uracil, adenine, guanine, and cytosine) and their UV-vis profiles were investigated (Fig. 9).

Fig. 9 UV-vis absorption spectra showing negligible interference due to different biomolecules in the recognition behaviour of H1 towards (a) ADP, and (b) NADH respectively.

As can be clearly seen from the UV-vis spectra, none of the biomolecules pose any interference to the recognition behaviour of the receptor in the form of H1. The effect of varying the pH on the recognition behaviour of H1 was investigated and the UV-vis spectrum was recorded (Fig. S6†). It is quite apparent that UV-visible profile of H1 remains unaltered in a wide pH range of 2 to 12. Hence, the proposed sensor is stable in this pH range and can be applied for use in samples of biological and environmental utility, without losing its sensitivity or selectivity. In order to investigate the salt effect on the recognition behaviour of H1, tetrabutylammonium perchlorate salt was added in increasing amounts (1–100 equiv.) to H1 in a 5 mL volumetric flask and the UV-visible spectra of the samples were recorded (Fig. S7†). It was perceived that even the presence of 100 equivalents of salt does not alter the response of H1. Hence, it can be said that the prepared nanohybrid complex (H1) can be used for the real sample analysis of NADH and ADP in samples of biological and environmental importance.

Spiked sample analysis

The prepared hybrid was scrutinized using spiked samples (A1–A4) of NADH. A1 and A2 were made by dissolving ADP and NADH, respectively. A3 and A4 were made by dissolving both NADH and ADP in the same solution to investigate the workability of H1 in the simultaneous estimation of NADH and ADP. The results obtained are given in Table 1.

Table 1 Analysis of the artificially made samples of NADH and ADP using H1

S.no	Sample name	Added conc. (nM)	Recovered conc. (nM)	Percentage recovery
1	A1 (ADP)	4.5	4.46 ± 0.03	99.1%
2	A2 (NADH)	3.7	3.64 ± 0.04	98.38%
3	A3 (ADP + NADH)	12.9	12.73 ± 0.06	98.68%
		6.8	6.63 ± 0.04	97.5%
4	A4 (ADP + NADH)	7.6	7.49 ± 0.03	98.55%
		14.8	14.48 ± 0.07	97.84%

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It is quite evident from Table 1 that H1 can be used for the real sample analysis of various spiked samples of analytical importance for NADH and ADP with an accuracy of more than 95%. The hybrid can also be successfully employed for the simultaneous estimation of NADH and ADP, without any loss in selectivity or sensitivity.

Conclusions

Receptor R1 was synthesized by the Schiff’s base condensation reaction of a calix[4]arene-based dipodal aldehyde and 3-(dimethylamino)-1-propylamine. The receptor was then subjected to organic nanoparticles (N1) and further to organic–inorganic nanohybrids (H1). The sensor activities of H1 were investigated by subjecting it to various biomolecules in aqueous media using UV-visible spectroscopy. It was perceived that the receptor worked well for the simultaneous determination of ADP and NADH even in the presence of each other as interferent molecules. The proposed sensor is stable in a wide pH range, hence it could be used for the simultaneous determination of the above mentioned biomolecules in various environmentally and biologically important aqueous samples.

Experimental

Materials and methods

All reagents viz. p-hydroxybenzaldehyde, 1,2-dibromoethane, p-tert butyl phenol, formaldehyde (37%), N,N-dimethylpropane-1,3-diamine, K₂CO₃ and NaOH (analytical grade) were purchased from Loba Chemie and used without further purification. Solvents such as diphenyl ether, ethyl acetate, and acetic acid were procured from SD Fine and were used as such, whereas acetonitrile (HPLC Grade) was obtained from Fisher Scientific and was further dried by distillation before use. All biomolecules viz. ADP, NADP, NADH, NAD, ATP, AMP, cytosine, adenine, guanine and uracil were obtained from Sigma Aldrich. De-ionized bi-distilled water was used for the preparation of organic–inorganic nanohybrids and solutions of all biomolecules.

UV-visible absorption spectra were recorded on a Spectroscan 30 spectrophotometer from Biotech Engineering Management Co. Ltd. (UK). The fluorescence measurements were performed on a RF-5301 PC spectrofluorophotometer from Shimadzu. The particle sizes and size distribution of the ONPs and AuNPs were determined using a Metrohm Microtac Ultra Nanotrac Particle Size Analyzer (dynamic light scattering). Transmission electron micrographs (TEM) were recorded on a Hitachi (H-7500) instrument working at 120 kV, with a resolution of 0.36 nm (point to point) and a 40–120 kV operating voltage. For the sample preparation, a carbon-coated copper grid (400-mesh) was used. Elemental analysis was carried out on a Fisons instrument (Model EA 1108 CHNO).

Synthesis of receptor 1

The calix[4]arene-based dipodal receptor (I) was synthesized as depicted in Scheme S1 (ESI†). The calix[4]arene-based dipodal aldehyde 1 was prepared by the reported method²² from p-tert butyl calix[4]arene and 2-(2-bromoethoxy) benzaldehyde (Scheme S1, ESI†). Receptor 1 was obtained in 70% yield, and was prepared by the Schiff’s base condensation reaction of 3-(dimethylamino)-1-propylamine and dipodal aldehyde 1. The receptor 1 obtained in 70% yield was white in color and was characterized using elemental analysis. Expected percentage: C = 77.66, H = 8.69, N = 5.03, O = 8.62; obtained percentage: C = 77.89, H = 8.46, N = 5.16, O = 8.54.

Synthesis of organic nanoparticles (N1) of receptor 1

Organic nanoparticles of receptor 1 (N1) were primed using a reprecipitation method. 1 mL of a stock solution of 1 (in tetrahydrofuran) was injected with a micro syringe at a steady rate into 100 mL de-ionized water under vigorous stirring. The solution obtained was sonicated for half an hour at constant temperature to ensure the formation of N1. The size distribution of the N1 nanoparticles formed was scrutinized using a particle size analyzer Dynamic Light Scattering (DLS) technique.

Synthesis of organic–inorganic hybrids (H1)

Organic–inorganic nanohybrids (H1) were prepared by mixing N1 in a particular ratio with the reduction product of HAuCl₄ using ascorbic acid. For this, 250 mL aqueous solutions of HAuCl₄ (1 mM) and ascorbic acid (1 mM) were prepared using de-ionized water from stock solutions of HAuCl₄ (10.0 × 10⁻³ M) and ascorbic acid (10.0 × 10⁻³ M). The solutions were equilibrated at ambient temperature for 2 hours and were then mixed along with N1 to obtain H1. The appearance of the color pink in the solution confirmed the formation of H1. There are various factors which control the size distribution of the hybrid nanoparticles in the solution such as the ratio of the three components i.e. gold, ascorbic acid and the nanoparticles, the order in which the reagents are added and physical conditions such as temperature. The methodology we have followed here is prompt and simple, yielding reproducible results and mono-dispersed nanoparticles.

Recognition studies

UV-visible spectroscopy was used for the recognition studies of the prepared H1. To a solution of H1 in a 5 mL volumetric flask were added 100 μL of various biomolecules (5 μM) (NADH, NADP, NAD, AMP, ATP, ADP, uracil, cytosine, adenine and guanine). The volumetric flasks were then shaken well and equilibrated for half an hour at temperature 25 ± 1 °C before recording the spectra. To confirm the binding, titrations were carried out with NADH and ADP for which small aliquots of both the biomolecules were added into 10 mL H1 in separate volumetric flasks. In order to evaluate the interference due to the presence of other biomolecules, interference studies were carried out. For this, UV-vis spectra were recorded for solutions of H1 containing NADH and ADP, with or without different biomolecules. In order to investigate the ability of the receptor to simultaneously determine ADP and NADH without having any interference due to each other when present in excess amounts, an experiment was performed in which one analyte was present in excess in a given volume of H1 and a titration was performed with the other analyte. The results showed no discernible interference of one analyte in the detection of the other. In addition, the effect of a high concentration of salt was studied by adding different concentrations of tetrabutyl ammonium perchlorate salts (0–100 equiv.). Experiments showing the response time were also carried out by recording the spectra of H1 at various concentrations with respect to time. A pH titration was also performed, so as to investigate the effect that varying the pH may have on the performance of H1.

Spiked sample analysis

To extend the use of H1 as a sensor and for real sample analysis, solutions of known concentrations (A1–A4) of NADH and ATP were made and scrutinized using the prepared H1.

Acknowledgements

This work was supported with a research grant from CSIR, New Delhi through a research project sanctioned to NK (Project No. 02(0216)/14/EMR-II). HK would like to acknowledge DST-INSPIRE for the fellowship.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18003a

‡ Both authors contributed equally.

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