The soft interactions of aminated SiO₂ nanoparticles with fluorescent partners: a multi-functional sensing platform with a signal amplification effect

Abstract

Signal amplification, known as the “molecular wire” effect, is greatly desirable and achieved by the electrostatic interactions of aminated SiO₂ nanoparticles (SiO₂–NH₂) with acidic fluorescent partners (NBTA). This explored sensing platform exhibited a multi-responsive capability toward external stimuli, including that of TFA/Et₃N, chloroauric acid, as well as the explosive compound PA.

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1. Introduction

All the time, the convenient detection of explosive aromatic compounds has drawn great interest due to district security.^1–5 Moreover, such explosive aromatic compounds were recognized as toxic contaminants for environmental pollution and harmful substances for living beings' health.^4,6,7 Therefore, the investigation into the enhancement of detection selectivity, sensitivity and economical superiority is of urgent demand but is still a great challenge.^4,8–14 To date, types of chemical and physical analytical methods have been applied to explore the sensing performance with a great deal diversity. Among them, fluorescence quenching has been widely employed and proven to be one of the most effective tools over neutron activation, X-ray diffraction, and ion mobility spectrometers as documented in the literature.^15–17 Among these, fluorescent conjugated polymers (FCPs) have recently been used successfully in explosive analyte detection. The representative studies documented the following: poly(phenyleneethynylene)s, Poly(phenylenevinylene)s, polyfluorenes, polycarbazoles, polyacetylenes, polythiophenes, and others.^12,18–22 Compared with small molecule fluorophores, FCPs were able to demonstrate an amplification phenomenon known as the “molecular wire” or “one point contact, multi-point response” effect, which was due to the fact that CPs are excellent electron donors, and their donor ability is enhanced by the delocalized π* excited state, which facilitates exciton migration and hence increases the electrostatic interaction between the polymer and electron-deficient nitro-aromatic analytes.²³ The effect is illustrated in Fig. 1. However, it is well known that for real-time monitoring of metal ions and explosives in environmental and security-related fields, their synthesis involves a multi-step process, results in unsatisfied yields, and is usually time and large cost consuming.^24,25 In addition, it is difficult to obtain polymers with controllable molecular weights with the anticipated molecular organization or structure. Furthermore, most fluorescent polymers comprise hydrophobic aromatic segments, which prevent them from the real applications due to their low solubility in commonly used organic solvents, and even in the aqueous phase.^26–28

Fig. 1 The schematic of molecular wire theory.

Therefore, it is necessary to explore a new sensory platform that is able to address those issues and still realize the “molecular wire” effect successfully. For supramolecular polymers, more molecular entities self-aggregate into an infinite network with a well-defined or ordered molecular organization through soft interactions such as H-bonding, π–π stacking, van der Waals interactions.²⁹ In addition, there are advantages that supramolecular polymers afford, including that they can be fabricated by a simple process, have varied pathways for fluorescence quenching and that they have the ability to detect a wide range of aromatic explosives at ppt levels in solution.

In recent years, we have succeeded in creating a group of novel triphenylamine (TPA) and tetraphenyl ethylene (TPE)-containing illuminant molecules.^30–34 In this study, we developed a simple but efficient supramolecular sensing system comprising NH₂ group fabricated nano SiO₂ as a proton acceptor and 3,3′,3′′-(nitrilotris(benzene-4,1-diyl))triacrylic acid (abbreviated as NBTA) as a proton donor, both of them organized into aggregated nano-composites spontaneously by an electrostatic attraction, leading to the formation of a hybrid stable fluorescent solution in EtOH (Fig. 2). This platform demonstrated a multi-responsive behavior towards different stimuli sources with a “one point contact, multi-point response” effect. The chemical stimuli sources, which have a stronger H-donating ability than NBTA has, can respond with sharp changes in the emission characteristics. In particular, to validate the effective sensing capability, we employed the more active, H-donating explosive substance PA herein, which prefers to anchor to SiO₂–NH₂ more than NBTA does. Once the fluorescent nanocomposites are incubated with PA for seconds, PA could replace NBTA, and thus the multi-point response could occur.

Fig. 2 The synthesis of NBTA and the interaction mode of SiO₂–NH₂ NPs with NBTA.

2. Result and discussion

As usual, TPA derivatives typically display aggregation-induced emission (AIE).³⁵ Therefore, we mixed different ratios of water and THF to test the emissive performance of NBTA. As can be observed from Fig. 3f, once the ratio of water reached 10% by volume, the fluorescent quenching definitely occurred. In the case of 90% water with 10% THF, the emission peak almost totally disappeared. This result verified NBTA as an aggregation-caused quenching (ACQ) molecule. Fig. 3a and b displayed that the powdered NBTA is not emissive; however, in the miscible solvent THF, NBTA emits a very strong blue light. The aggregation ability was evidenced by the H–H 2D NOESY spectrum (Fig. 4a), which indicated obvious cross-peak signals between the protons of the C=C bond of crylic.^36,37 Moreover, SEM images displayed a thick film folded in the silica substrate clearly (Fig. 4b). Therefore, all these results confirmed the favorable aggregation ability of NBTA in solution.

Fig. 3 (a, b) NBTA in powder form under room light (left) and UV light illumination at 365 nm (right). (c, d) Images of NBTA in THF taken under room light (left) and UV illumination at 365 nm (right). (e) Emission spectra of NBTA in different solvents (solution concentration: 30 M, λ_em = 370 nm). (f) Emission spectra of NBTA in THF/water mixtures (solution concentration: 30 M, λ_em = 370 nm).

Fig. 4 (a) NOESY NMR spectrum of NBTA in CDCl₃. (b) SEM image of NBTA.

SiO₂–NH₂ NPs possess a broad absorption band in the UV-vis region. This optical property allows SiO₂–NH₂ NPs to act as efficient acceptors for most fluorophores. In particular, the overlapped absorption spectrum of SiO₂–NH₂ NPs and the fluorescence emission spectrum of NBTA is very necessary to promote Förster resonance energy transfer (FRET), as shown in Fig. S1, ESI.† Therefore, we inferred that the decreasing PL intensity of the composites is ascribed to the occurrence of FRET between the SiO₂–NH₂ NPs and NBTA when the concentration of SiO₂–NH₂ is below 15 × 10⁻² g L⁻¹. Evidently, the PL intensity strengthens greatly by increasing the concentration of the SiO₂–NH₂ NPs after this point (Fig. 5a). Therefore, we postulated that the increased PL intensity originated from the more and more dispersed NBTAs in the SiO₂–NH₂ NP networks and the different emissive performances of NBTA and deprotonated NBTA. Thus, to verify the difference between NBTA and deprotonated NBTA in optical behaviours, theoretical calculations were employed to optimize the conformations and the molecular orbital energy levels using DFT/B3LYP/6-31G*(d) (Gaussian09). From Fig. 6, we could observe that in contrast to NBTA, the deprotonated NBTA has a lower orbital energy and more narrow energy gap. From the SEM imagines of SiO₂–NH₂ and the SiO₂–NH₂&NBTA composite (Fig. 7), we can confirm that the NH₂ modified SiO₂ displays more aggregated spheres, on the 50–60 nm scale. In sharp contrast, more crushed and smaller nanoscale objects were crowded together in the case of the SiO₂–NH₂&NBTA composite. This result verified that the aggregated SiO₂–NH₂ particles are dispersed when NBTAs are introduced.

Fig. 5 (a) Emission spectra of NBTA at different SiO₂–NH₂ NP concentrations in EtOH (NBTA concentration: 30 M). (b) Plots of maximum emission intensity (I/I₀) and wavelength (λ_em = 370 nm) of NBTA in EtOH. (c) Images of NBTA at different SiO₂–NH₂ NP concentrations in EtOH. SiO₂–NH₂ NP concentration: 0–0.6 g L⁻¹, NBTA concentration: 30 M, the fluorescence quantum yields in ethyl acetate (Φ_f) are determined by using quinine sulfate as a standard (Φ_f = 45% in 0.1 M sulphuric acid, excitation wavelength: 369 nm).

Fig. 6 The optimized molecular orbital amplitude plots of HOMO and LUMO energy levels of NBTA (left) and NBTA anion (right).

Fig. 7 (a) The SEM imagines of SiO₂–NH₂ and (b) SiO₂–NH₂&NBTA composite.

The interaction modes between –COOH and –NH₂ are able to form ammonium groups (–NH⁴⁺), which assemble with carboxylate anions (–COO⁻) through an electrostatic interaction. Accordingly, this soft mode could be destroyed by TFA, and then recovered by Et₃N. As our test results displayed that these acid/base triggered PL emissions and decays are completely reversible by adding TFA and Et₃N. Notably, the elegant reversibility in the optical performance was realized for at least three of the runs (Fig. 8). To the best of our knowledge, other pH-responsive composite materials reported to date can only be switched in one direction by the addition of either acid or base.^38,39

Fig. 8 (a) Emission spectra of SiO₂–NH₂&NBTA in the presence of TFA. (b) Emission spectra of SiO₂–NH₂&NBTA&TFA in the presence of Et₃N. (c) The I/I₀ changes when SiO₂–NH₂&NBTA is triggered by TFA/Et₃N. (d) Images of the SiO₂–NH₂&NBTA solution, when adding TFA, then charging Et₃N.

The TFA/Et₃N responsive capability of SiO₂–NH₂&NBTA nano-composites inspired us to check if the new electrostatic interaction pattern could be reformed for some chemical species, which have a stronger electron donating ability. As documented, metal cations are susceptible to being absorbed and confined on the surface of amino-modified SiO₂ colloids due to their strong chemical affinity with the amino groups.⁴⁰ Therefore, we incubated the SiO₂–NH₂&NBTA system with different concentrations of chloroauric acid in EtOH. The PL measurements indicated the real-time fluorescent quenching behavior (Fig. 9). Notably, when 0.1 equivalent chloroauric acid towards NBTA was added, the quenching efficiency of SiO₂–NH₂&NBTA increased by up to 76%. This phenomenon exhibited a great amplification effect on our sensing system in the field of heavy metal salts recognition. Moreover, we were anxious to investigate what happens to Au⁺ in the fluorescent quenching process. Thus, a morphology study was carried out by TEM analysis. Astonishingly, nanoscale gold particles at 2–5 nm, embedded in the SiO₂–NH₂&NBTA network, can be observed clearly (Fig. 10). The reductant-free gold nanoparticle formation process promoted us to carry out the control experiment, wherein the incubation of SiO₂–NH₂ with Au⁺ solution was performed in the absence of NBTA under the same preparation conditions. Then, TEM analysis of the control experiment displayed no nanoscale objects that could be found. Therefore, we concluded that NBTA probably acts as reductant and stabilizer for the gold nanoparticle formation. For the further applications of gold nanoparticles, we utilized p-nitrophenol as a substrate, NaBH₄ as a reductant, and a gold nano-particle containing solution as a catalyst. The real-time UV analysis monitored the reaction process, and the results demonstrated the excellent catalytic ability of the gold nanoparticles towards p-nitrophenol (p-NP) reduction in the presence of the gold nanoparticle catalyst (0.1 eq.) in EtOH. With NaBH₄ (5 eq.) in water, p-nitrophenol (1.5 × 10⁻³ mmol) could be converted into p-aminophenol (p-AP) almost completely within 30 min (Fig. 10).

Fig. 9 Emission spectra of SiO₂–NH₂&NBTA at different chloroauric acid concentrations in EtOH. Inset shows the blank sample and the mixture of blank sample with chloroauric acid.

Fig. 10 (a) Transmission electron microscope (TEM) of gold nanoparticles. Inset shows the selected area electron diffraction (SAED) pattern exhibited a crystalline nature of Au. (b) The UV spectrum of the reduction process of p-nitrophenol by gold nanoparticles.

The good fluorescence of the supramolecular composite with a distinct “one point contact, multi-point response” effect encouraged us to evaluate their potential application for explosive detection. In specific, SiO₂–NH₂&NBTA aggregates in an EtOH solution were utilized as detectors, and the commercially available picric acid (PA) was adopted as an explosive model. Due to the fact that PA has a super energy release power, even more than TNT does, therefore, it is regarded as a more explosive and dangerous substance.^41,42 Moreover, as the term implied, PA is very preferential to producing protons (pK_a = 0.38) and therefore PA is considered a very acidic proton donor. Fig. 11 exhibited the PL spectra of SiO₂–NH₂&NBTA aggregates with the addition of PA. Apparently, the fluorescent intensity decreased dramatically when PA was gradually added into the detector solution. What should be noted is that once PA was added to a 0.1 equivalent amount to NBTA, 85% quenching efficiency could be obtained. The detection limits of the aggregates towards PA could reach 30 ppb (D_L = 3 × 10⁻⁶ M × 0.01 equiv. = 3 × 10⁻⁸ M = 30 ppb). To verify the selectivity for PA detection, several possible interfering analytes were employed. We could see that only –OH substituted nitro-compounds displayed a significant interference effect, including 2,6-dinitrophenol, 3-nitrophenol, 4-nitrophenol, and 2-nitrophenol. In sharp contrast, the quenching efficiency was below 5% in the case of the more electron deficient 2,4-dinitrobenzaldehyde. Similar effects also could be obtained in other samples of halogenated nitro-compounds (Fig. 12). Therefore, a preliminary conclusion could be made that the stronger the proton donating ability of the analyte, the higher quenching signal the detector could release.

Fig. 11 (a) Emission spectra of SiO₂–NH₂&NBTA at different PA concentrations. Inset shows the blank sample and the mixture of blank sample with PA. (b) The plot of detection limit analysis of SiO₂–NH₂&NBTA towards PA.

Fig. 12 Selectivity graph of SiO₂–NH₂&NBTA towards various nitro derivatives.

To validate the sensing mechanism, a variety of controlled PL analyses were conducted. As Fig. 13a displayed, more than 10 plots produced two totally different results. The PL intensity of SiO₂–NH₂&NBTA was almost 7 fold higher than NBTA under the same concentration of NBTA. Therefore, SiO₂–NH₂ acted as a robust promoter to extraordinarily enhance the PL intensity of the composite. In addition, the grafted NH₂ groups on the surface of the SiO₂ nanoparticles played an important role in elevating the PL performance and sensing capability, which was due to the fact that if naked SiO₂ nanoparticles were incubated with NBTA, or other analytes, only negligible PL intensity changes could be found. Therefore, SiO₂–NH₂ nanoparticles were indispensible in the realization of a “one point contact, multi-point response” effect. Further optical analysis also verified the different FRET effect in the presence of SiO₂–NH₂&PA or PA, as shown in Fig. 13b. Their absorption spectra have very different overlap areas with the PL spectrum of NBTA. Notably, the FRET effect exhibited more efficiency in the case of SiO₂–NH₂&PA, but not in the presence of PA only. Therefore, SiO₂–NH₂ nanoparticles displayed an amplification effect in the following sensing sequence. Moreover, we found that after fluorescent quenching occurred, all the samples showed a PL emission peak at 525 nm. Evidently, 525 nm was designated as the emission wavelength of NBTA. Therefore, we postulated that once one point is attacked, the supramolecular networks collapsed instantaneously, and then NBTAs are released, leading to the aggregation of NBTAs and the appearance of emission peak at 525 nm.

Fig. 13 (a) The PL spectrum of the SiO₂–NH₂&NBTA nanocomposite (g) and the controlled samples (a. NBTA b. NBTA + Au³⁺ c. NBTA + PA d. NBTA + SiO₂ e. NBTA + SiO₂ + Au³⁺ f. NBTA + SiO₂ + PA h. NBTA + SiO₂–NH₂ + Au³⁺ i. NBTA + SiO₂–KH550 + PA). (b) The overlapped UV spectra of PA and SiO₂–NH₂&PA with the PL spectrum of NBTA.

3. Experimental

Synthesis of SiO₂–NH₂ nanoparticles

SiO₂–NH₂ nanoparticles were obtained by the documented method.⁴³

Synthesis of trimethyl 3,3′,3′′-(nitrilotris(benzene-4,1-diyl))triacrylate (TNBT)

Tris(4-iodophenyl)amine (2.4 g, 5 mmol), methylacrylate (18 mmol, Pd(OAc)₂) (27 mg, 0.12 mmol), 1,3-bis(diphenylphosphino)propane (DPPP, 103.1 mg, 0.25 mmol) and Et₃N (1.8 g, 18 mmol) were added to 10 mL of dry dimethylformamide (DMF) and stirred for 48 h at 100 °C under a N₂ atmosphere. The reaction process was monitored by the disappearance of the tris(4-iodophenyl)amine. Then, 20 mL of water was charged, and the solution was extracted with CH₂Cl₂ (3 × 10 mL). The organic phases were dried with anhydrous magnesium sulfate, filtered, and the CH₂Cl₂ solution was concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (300–400 mesh) with a mixture of ethyl acetate and petroleum ether as the eluent (1 : 100 by volume), leading to the purified TNBT 2.24 g, yield: 90%.

Synthesis of 3,3′,3′′-(nitrilotris(benzene-4,1-diyl))triacrylic acid (NBTA)

TNBT (2.49 g, 5 mmol) and KOH (1.12 g, 20 mmol) were added into the mixed solvent (THF : distilled water = 6 : 1, 35 mL). Warming at 95 °C for 12 h, the solution was evaporated to remove THF under reduced pressure. Charging the HCl solution into the aqueous phase, the pH value was adjusted to 1.5 and a green solid precipitated, was filtered, and then dissolved into acetone. Subsequently, MgSO₄ was added to the solution, removing the small fraction of water. The solution was then filtered and evaporated to obtain the purified NBTA 2.1 g, yield: 95%. Mp > 300 °C; ¹H NMR (600 MHz, CDCl₃) δ 7.65 (d, J = 7.8 Hz, 2H), 7.55 (d, J = 15.8 Hz, 1H), 7.06 (d, J = 7.8 Hz, 2H), 6.43 (d, J = 15.9 Hz, 1H). ¹³C NMR (100 MHz, CDCl₃) δ 170.9, 145.6, 140.8, 129.3, 127.1, 121.4, 116.9 ppm. IR (KBr): 1684, 1592, 1507, 1397, 1318, 1270, 1176, 1118, 869, 819, 517 cm⁻¹.

The preparation of SiO₂–NH₂&NBTA nanocomposites

NBTA (13.35 mg, 0.03 mmol) was dissolved in 100 mL of ethanol (Con. 3 × 10⁻⁴ M) and SiO₂–NH₂ (100 mg) was dispersed in 100 mL ethanol (Con. 1 g L⁻¹), respectively. Both the NBTA solution (1 mL) and the SiO₂–NH₂ solution (4 mL) were mixed and 5 mL of ethanol was then charged into the mixture, which was used as the standard solution in the following detections.

4. Conclusions

In this study, we developed a robust sensing platform, which was established by the soft electrostatic interactions of aminated SiO₂ nanoparticles (SiO₂–NH₂) with acidic fluorescent partners (NBTA). We found that the emission capability of NBTA could be manipulated by SiO₂–NH₂ completely. At lower concentrations of SiO₂–NH₂ (below 0.13 g L⁻¹), Förster resonance energy transfer (FRET) induces fluorescence quenching. Comparably, elevating the concentration of SiO₂–NH₂ from this point, the nanocomposites demonstrated better emissive capability. In contrast to NBTA, the fluorescence quantum yield (Φ_f) increased from 11% to 45% when the concentration of SiO₂–NH₂ was ranged from 0 to 0.6 g L⁻¹. In addition, this explored platform exhibited a stable and multi-sensitive ability with a signal amplification effect towards external stimuli, including of TFA/Et₃N, chloroauric acid, as well as the explosive compound PA. Further application of this efficient sensing system is ongoing in our research program.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (no. 21202133, and 21361023). We thank the Key Laboratory of Eco-Environment-Related Polymer Materials (Northwest Normal University) and the Ministry of Education Scholars Innovation Team (IRT 1177) for financial support. We also thank the Analytical and Testing Center of Northwest Normal University for related analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14391a

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