Near-infrared photoluminescence enhancement of N-acetyl-L-cysteine (NAC)-protected gold nanoparticles via fluorescence resonance energy transfer from NAC-stabilized CdTe quantum dots

Abstract

Water-soluble N-acetyl-L-cysteine-protected gold nanoparticles (NAC-AuNPs) and NAC-stabilized cadmium telluride quantum dots (NAC-CdTeQDs) have been synthesized. The near-infrared (NIR) photoluminescence (PL) of NAC-AuNPs in various aqueous pH phosphate buffers has been studied in the presence of NAC-CdTeQDs with different core sizes. The NIR PL intensities of 1.7 nm and 2.4 nm sized NAC-AuNPs increases with NAC-CdTeQDs increase. The PL enhancement for 2.4 nm NAC-AuNPs is larger than 1.7 nm NAC-AuNPs and also increases with the core size of NAC-CdTeQDs from 1.9 to 3.2 and then to 3.7 nm. In addition, the PL enhancement effect initially increases with pH and then drops with a further increase in pH, attributed to the coulombic repulsion between the same negatively charged NAC-AuNP and NAC-CdTeQD molecules at pH ≥ 7. At pH 4.0, NAC-CdTeQDs can show quenching on NAC-AuNPs. In contrast, the PL of NAC-CdTeQDs in the visible region is strongly quenched by the presence of NAC-AuNPs. The quenching efficiency decreases with the core size of NAC-AuNPs from 1.7 to 2.4 and then to 2.9 nm attributed to the decrease in the collision frequency of larger core NAC-AuNPs with NAC-CdTeQDs. Furthermore, the quenching effect increases with the decrease of pH as the coulombic repulsion between NAC-CdTeQDs and NAC-AuNPs is minimized under a low pH environment such that the intermolecular distance between the interacted NPs is shortened. The quenching of yellow and red is smaller than green NAC-CdTeQDs by NAC-AuNPs at pH 6.0 but larger than green ones at pH 7.0. The results demonstrate that the interaction between NAC-AuNPs and NAC-CdTeQD is mainly governed by the fluorescence resonance energy transfer model and the collision frequency of the AuNPs and QDs.

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Introduction

Nanomaterials actively being pursued including nanoparticles (NPs),¹ nanowires,² nanoshells,³ nanotubes,⁴ and many others⁵ have attracted widespread attention because of their specific features that differ from bulk materials. Among these, NPs are the most widely studied and have many potential applications.⁶ Metal–semiconductor NPs with specific size- and shape-dependent optical and electrical behaviors, in particular, are of growing importance in the field of optics, electronics, catalysis, magnetic storage, and biophysics in recent years.^7–9 Monolayer-protected NPs are stable, structurally well-defined even in the dried state, which coalesce classical metal colloid chemistry with monolayer self-assembly methods to form metallic core/organic shell particles with diameters of less than 5 nm.¹⁰ The size-dependent optical and electrochemical charging properties of NPs have been described in detail.^11,12 Gold (Au) NPs with 1–3 nm core diameters are attracting more special interests as they display strong size-dependent emission properties. In the case of large NPs (radius > 2 nm), weak plasmon emission in the visible region can be observed.¹³ As AuNPs become smaller, luminescence bands can be observed in the near-infrared (NIR) region,¹⁴ which are strongly dependent on the nature of the surface ligand.¹⁵ AuNPs are believed to have far-reaching applications using these systems with NIR luminescence bands.¹⁶

Luminescence semiconductor nanocrystals (NCs), also called quantum dots (QDs), exhibit a number of advantageous features including broad absorption cross-sections, tunable emission profiles, high molar extinction coefficients, resistance to photobleaching, and long photoluminescence (PL) lifetimes. Especially, QDs show size-dependent emission spectra over most of the visible region and can be efficiently excited at any wavelength below the absorption band edge from the visible to UV ranges.¹⁷ For example, cadmium selenide (CdSe) QDs may be size-tuned to emit in the 450–650 nm range whereas cadmium telluride (CdTe) QDs can emit in the 500–750 nm range.¹⁸ Due to these superior intrinsic optical properties, water-soluble QDs are far more suitable for direct use in biological analysis comparing with conventional organic fluorophores and dyes. As such, intensive researches have focused on colloidal CdTe, CdSe, cadmium sulfide, indium phosphide, and lead sulfide QDs, etc.

Furthermore, QDs have been favorably used in fluorescence (or Förster) resonance energy transfer (FRET)-based studies.^19–22 FRET is a process in which energy is transferred from a donor molecule in the excited state to an acceptor molecule in the ground state via a resonant, near-field dipole–dipole interaction when molecules are in close proximity to each other and there is sufficient overlap between the donor emission and the acceptor absorption spectra. QDs were proven to be very effective FRET donors with an array of organic dye acceptors, and several FRET-based biosensing assemblies utilizing QDs were demonstrated in the past few years. QDs serving as energy donors in luminescence resonance energy transfer have been successfully used for detecting maltose,²³ protease,¹⁸ toxins,²⁴ lactamase,²⁵ and so forth.¹⁹ At the same time, NPs (especially AuNPs) as an efficient fluorescent quencher open new perspectives in FRET systems owing to their high extinction coefficients as well as broad absorption spectra in the visible light range that can overlap with the emission wavelengths of QD donors. It was proposed that the use of AuNP as an energy acceptor extends the effective energy resulting in a high energy transfer efficiency compared to other organic quenchers.^26,27 Therefore, this ideal donor–acceptor FRET pair (QDs–AuNPs) has been favorably adopted in bioanalytical applications.^28–30 To use FRET-based system much more widely, of course, theory researches on interactions between QDs and AuNPs have been studied.³¹ The response of a hybrid nanostructure molecule consisting of a QD and a NP subject to an applied electric field has been discussed.³² A detailed characterization of the PL quenching of CdSe–ZnS core–shell QDs by proximal AuNPs was provided using a rigid variable-length polypeptide as a bifunctional biological linker (QD–peptide–Au-NP conjugates).²⁶ Wargnier et al. described the nanoscale of water-solubilized, oppositely charged QDs and AuNPs and made it possible to develop FRET-based sensors with a donor quenching efficiency close to 100%.³³ Besides so many studies and applications on quenching effects, there were only a few discussions on luminescence enhancements. For instance, Kulakovich et al. studied the enhancement of luminescence of CdSe–ZnS core–shell QDs on Au colloids as a function of QDs–metal NP distance.³⁴ However, most of these investigations aimed at probing the nature of the interactions between luminescent QDs and AuNPs^17,35 embedded in controlled assemblies, and how the interactions could affect the optical properties of these assemblies. Although the fluorescence quenching of CdTe or CdSe NCs by AuNPs has been discussed,^36,37 most of their work focused on the electrostatic interactions of oppositely charged surface-attached ligands. Studies on the same charged surface ligands, pH and core size effects of NCs and/or NPs are still scare.

This work represents an initial attempt to discuss a simple FRET-based system consisting of negatively charged N-acetyl-L-cysteine (NAC)-protected AuNPs (NAC-AuNPs) and NAC-stabilized CdTeQDs (NAC-CdTeQDs) in aqueous solutions under various pH environments. NAC has excellent biocompatibility, is nontoxic, user and environmentally friendly, inexpensive, stable, nonvolatile, and inodorous, and has good water-solubility.³⁸ It has been employed as an excellent water-soluble protected/stabilized ligand for NCs.^38,39 The surface-attached NAC ligands on the NCs semiconductor/metal cores can render NCs water-solubility and also possibly reduce NCs toxicity. In this work, PL of NAC-AuNPs is observed in the NIR range where most substances, e.g., water and biological molecules have the lowest background absorptions. Interestingly, NAC-AuNPs show good PL enhancement when interacting with NAC-CdTeQDs. By contrast, the NAC-CdTeQDs display PL quenching in the visible light region when interacting with NAC-AuNPs. To our knowledge, this is a first report on studying the energy transfer processes between the same negatively charged NAC-AuNPs and NAC-CdTeQDs fluorometrically in the NIR and visible light regions. A throughout investigation of the pH and core size effects on the energy transfer is reported. Our work is anticipated to be a useful analytical technique using both fluorescence quenching and enhancement which may have importance in applications and fundamental research.

Results and discussion

Spectral characteristics of NAC-protected AuNPs and NAC-stabilized CdTeQDs

NAC was well attached to the Au and CdTe cores via the strong Au or CdTe cores–S covalent bonds as shown in Scheme 1. In this work, three sizes of NAC-AuNPs with 1.7, 2.4 and 2.9 nm were synthesized respectively. Their sizes were determined by TEM and reported in our previous work.⁴⁰ The TEM results of the mixture of NAC-AuNPs and NAC-CdTeQDs were shown in Fig. 1. The particles size distributions of NAC-CdTeQDs (deep color nanoparticle, yellow circle) and NAC-AuNPs (light color nanoparticle, red circle) are 2.05–2.75 nm and 3.45–3.95 nm with average diameters of 2.40 ± 0.20 nm and 3.70 ± 0.25 nm, respectively. Both NAC-AuNPs and NAC-CdTeQDs are mostly of spherical morphology and disperse rather evenly on the TEM grid surface. There is no overlap and link between NAC-AuNPs and NAC-CdTeQDs, which illustrate that no new chemical bonds were generated between NAC-AuNPs and NAC-CdTeQDs. Fig. 2A depicts the UV-visible absorption spectra of the different sized NAC-AuNPs. These peaks were normalized at 260 nm to remove effects of concentration differences to compare the shape and band position. It can be observed that the smallest NAC-AuNPs (1.7 nm) demonstrate a sharper decrease in absorbance range than that of larger NAC-AuNPs (2.4 and 2.9 nm). Furthermore, the surface plasmon band at 520 nm becomes more prominent and gradually red-shifted with the increase in particle size. When the NAC-AuNPs were excited at 400 nm, broad corrected NIR PL bands centered at 780 and 813 nm for 1.7 and 2.4 nm NPs with wide spectral width of 200 nm, respectively were identified and shown in Fig. 2B, indicating that larger AuNPs will produce more red-shift PL spectrum. Unfortunately, when the NAC-AuNP grows to 2.9 nm size, no PL band is found in the visible and NIR regions, inferring that large NAC-AuNP own small quantum efficiency.

Scheme 1 Interactions between NAC-AuNPs and NAC-CdTeQDs species under different pH conditions.

Fig. 1 (A) TEM image of the mixture of NAC-AuNPs and NAC-CdTeQDs and particle size distribution histogram of (B) NAC-AuNPs and (C) NAC-CdTeQDs.

Fig. 2 (A) Normalized absorption spectra of (1) 1.7 nm, (2) 2.4 nm, and (3) 2.9 nm, and corrected PL spectra of (4) 1.7 nm and (5) 2.4 nm sized NAC-AuNPs in 0.10 M pH 7.0 PBS. (B) Normalized absorption spectra of (1) 1.9 nm, (2) 3.2 nm, and (3) 3.7 nm, and corrected PL spectra of (4) 1.9 nm, (5) 3.2 nm, and (6) 3.7 nm sized NAC-CdTeQDs in 0.10 M pH 7.0 PBS buffer. The absorption and PL spectra are normalized at their λ_abs and λ_em, respectively. The inset displays the luminescence images of the 1.9 nm (left, green), 3.2 nm (middle, yellow) and 3.7 nm (right, red) NAC-CdTeQDs under UV light (λ = 254 nm) illumination.

There is a similarity on the PL properties of NAC-AuNPs and NAC-CdTeQDs, which are very sensitive to changes in the core size. Herein, three representative NAC-CdTeQDs samples were prepared, which the sizes of NAC-CdTeQDs were controlled by the reaction time. The absorption and PL spectra of NAC-CdTeQDs are shown in Fig. 2B. The NAC-CdTeQDs reacting at 30, 40 and 50 min indicate the well-known first excitonic absorption peaks (λ_abs) at 485, 543 and 606 nm, respectively. The difference in their absorption spectra is attributed to the different core sizes whose values can be obtained from their λ_abs:⁴¹

D (nm) = 9.8127 × 10⁻⁷λ_abs³ − 1.7147 × 10⁻³λ_abs² + 1.0064λ_abs − 194.84

where D (nm) is the size of a given NC sample. The calculated results illustrate that the three types of NAC-CdTeQDs are 1.9, 3.2 and 3.7 nm. The 1.9 nm NAC-CdTeQDs exhibits an maximum emission peak (λ_em) at 518 nm. When NAC-CdTeQDs increases to 3.2 nm and then to 3.7 nm, the λ_em is bathochromatically shifted to 546 nm and then to 640 nm using an excitation wavelength (λ_ex) of 400 nm. In summary, larger NAC-CdTeQDs core size will produce broader and red-shift absorption and PL spectra. What's more, different sized NAC-CdTeQDs display different colors under UV light (λ = 254 nm) irradiation as shown in the inset of Fig. 2B. They are green, yellow and red, which are henceforth denoted as green, yellow and red NAC-CdTeQDs, respectively. The spectral width of NAC-CdTeQDs PL were about 40–50 nm which are much smaller than that of NAC-AuNPs.

Effect of pH on NAC-AuNPs and NAC-CdTeQDs

The PL spectra of NAC-AuNPs under various pHs (4.0–9.0) does not change (not shown). However, NAC-CdTeQDs possess pH-sensitive. Fig. 3 displays the pH effect on the PL of the three sized NAC-CdTeQDs. The PL intensity increases with the pH increase. For the green NAC-CdTeQDs, the PL intensity is weaker at lower pH values. The intensity tardily increases from pH 4.0 to 5.5 and then begins to rise rapidly above pH 5.5. Since NAC is a weak organic acid with pK_a 3.2,⁴² the free ligands will be half-deprotonated at pH 3.2. The surface-attached NAC determines the surface chemistry of QD. When the free ligands are attached to the QD core, they will render water-solubility and weak acidity for the QDs. However, the average pK_a of QDs is generally higher than its free ligand. It is obvious that NAC-CdTeQDs turns to be strongly emissive when the surface-attached NAC is deprotonated at higher pH. By contrast, when the surface-attached NAC is protonated, the emission of NAC-CdTeQDs is completely quenched at lower pH values. The relative PL intensity is 0.5 when pH 7.2, indicating that the average pK_a of the green NAC-CdTeQDs is approximately equal to 7.2 which is about four units higher than the free NAC ligand. This also implies that the acidity of the NAC-CdTeQDs becomes about four orders of magnitude weaker than its free ligand in aqueous solution. The pH effect on the PL intensity of the yellow and red NAC-CdTeQDs behaves similarly. Our results demonstrate that smaller NAC-CdTeQDs (1.9 nm) possesses smaller average pK_a 7.2 as compared to larger NAC-CdTeQDs (3.2–3.7 nm) having average pK_a 7.5–7.6. However, this difference is very small when the QDs have comparable size. The QDs may be protonated at lower pH and destabilize the QD–ligand complex and eventually decrease the QDs PL intensity. On the other hand, the carboxylic acid moiety of NAC may be deprotonated to carboxylate anion at higher pH and the negative charges on the surface of QDs repel each other and no aggregation. This interaction is beneficial for stabilizing QDs and leads to higher PL intensity.

Fig. 3 pH effect on the luminescence intensity of the 12.5 μM solutions of green (red line), yellow (blue line) and red (cyan line) emission NAC-CdTeQDs. The inset depicts the corrected luminescence spectra of green, yellow and red emission NAC-CdTeQDs under various pHs (4–9).

PL enhancement of NAC-AuNPs

Interaction between NAC-AuNPs and NAC-CdTeQDs is provided by the PL behaviors illustrated in Scheme 1. For NAC-AuNPs, the NIR PL intensity increases when they collide with NAC-CdTeQDs. By contrast, the visible PL intensity quenches when NAC-CdTeQDs interact with NAC-AuNPs (vide infra), i.e., the PL quenching of NAC-CdTeQDs by NAC-AuNPs while PL enhancement of NAC-AuNPs by NAC-CdTeQDs. Since there is sufficient donor–acceptor spectral overlap for efficient energy transfer between NAC-CdTeQDs and NAC-AuNPs (Fig. 2), which would result in concurrent PL quenching of the donor and enhancement of the acceptor. FRET between semiconductor NCs has been recently reviewed.⁴³ The energy transfer is much largely determined by the distance between the donor and acceptor NCs and the spectral distributions of the emission of the donor and the absorption cross-section of the acceptor. The emission spectrum of the donor NC should overlap optimally with the absorption spectrum of the acceptor NC and is an important prerequisite for efficient energy transfer.

Fig. 4A shows the NIR PL enhancement of 2.4 nm NAC-AuNPs at pH 6.0 with the increase of the concentration of green NAC-CdTeQDs. As the PL spectrum (centered at 518 nm ranging 450–600 nm) of green NAC-CdTeQDs (magenta line in Fig. 2B) overlaps well with the absorption band (300–800 nm) of 2.4 nm NAC-AuNPs (red line in Fig. 2A), it is possible that FRET occurs in a donor–acceptor system composed of NAC-CdTeQDs as a donor and NAC-AuNPs as an acceptor. NAC-CdTeQDs behaves as a light-enrichment system via FRET which increases the excitation efficiency for NAC-AuNPs. In order to probe the possibility of forming stable NAC-AuNPs/NAC-CdTeQDs aggregation or compound, the absorption spectra of the NAC-CdTeQDs with and without NAC-AuNPs were displayed in Fig. S1 (ESI†). The absorption profiles of the NAC-CdTeQDs are almost the same in the presence and absence of NAC-AuNPs, indicating that there is no aggregation between NAC-AuNPs and NAC-CdTeQDs.

Fig. 4 PL spectra of 2.4 nm NAC-AuNPs (0.10 mg mL⁻¹) in 0.10 M PBS at (A) pH 6.0 and (B) pH 4.0 with the increase in concentration of green NAC-CdTeQDs: (1) 0.00, (2) 1.25, (3) 3.13, (4) 6.25, (5) 9.38, and (6) 12.5 μM. (C) The relative change (F − F_o)/F_o in PL intensity of 2.4 nm NAC-AuNPs against the concentration of green NAC-CdTeQDs under different pHs PBS (0.10 M). The PL intensities was recorded at λ_ex/λ_em of 400/813 nm.

Fig. 4B depicts the effect of green NAC-CdTeQDs on the PL spectrum of NAC-AuNPs at pH 4.0. Interestingly, the PL of NAC-AuNPs is quenched by the green NAC-CdTeQDs. At low pH (<4), the vast majority of NAC-AuNPs and NAC-CdTeQDs are protonated. As both NAC-AuNPs and NAC-CdTeQDs are neutral, they can be close vicinity and then possibly form aggregation. What is more, the NAC-CdTeQDs have no PL (Fig. 3) which will not promote FRET. These factors lead to quenching effect on NAC-AuNPs. Fig. 4C depicts the plots of change in PL intensity of 2.4 nm NAC-AuNPs against concentration of green NAC-CdTeQDs at various pHs. The change in PL intensity is defined as (F − F_o)/F_o, where F_o and F are the PL intensities of NAC-AuNPs in the absence and presence of NAC-CdTeQDs, respectively. At pH 5–9, the 2.4 nm NAC-AuNPs shows PL enhancement while it is quenched at pH 4.0. The enhancement effect is the largest at pH 6.0 and decreases with the further pH increase. In order to explain this phenomenon, it is essential to understand the different NAC-AuNPs and NAC-CdTeQDs species in pH buffer. There are four types of NAC-NPs in the solution of this system: the neutral NAC-AuNPs, anionic NAC-AuNPs, neutral NAC-CdTeQDs, and anionic NAC-CdTeQDs as illustrated in Scheme 1. The pK_a of NAC-AuNPs and NAC-CdTeQDs are 4.1 (ref. 42) and 7.2, respectively. At pH 4.0, NAC-AuNPs are approximatively dissociated to equal numbers of neutral and anionic species. They interact with the neutral NAC-CdTeQDs which have no PL and will not promote FRET in this system (routes 1 and 2). At pH 5.0, neutral NAC-AuNPs are partially transformed into anionic species. More neutral and less anionic NAC-AuNPs also interact with neutral NAC-CdTeQDs resulting in slight PL enhancement (routes 1 and 2). As the pH increases to 6.0, more anionic NAC-AuNPs and some anionic NAC-CdTeQDs species are formed which will promote PL enhancement (routes 2 and 3). However, when the pH is too high (9.0), all the NAC-AuNPs and NAC-CdTeQDs are deprotonated to anionic NAC-AuNPs and NAC-CdTeQDs resulting in more repulsion between the NAC-AuNPs and NAC-CdTeQDs (route 3). As such, the intermolecular distance between NAC-AuNPs and NAC-CdTeQDs increases with a concomitant decrease in the FRET-based interaction between the NAC-AuNPs and NAC-CdTeQDs as the pH increases. Finally, the degree in PL enhancement decreases with pH increases (>6).

Fig. 5 displays the effect of NAC-CdTeQDs on the PL enhancement of NAC-AuNPs at pH 7.0. The PL intensities of 2.4 nm (Fig. 5A, λ_em = 813 nm) and 1.7 nm (Fig. 5B, λ_em = 780 nm) NAC-AuNPs increase with the addition of NAC-CdTeQDs. The smaller NAC-CdTeQDs, the larger PL enhancement effect on NAC-AuNPs, i.e., green > yellow > red. First, the absorption spectrum of NAC-AuNPs is better overlapped with the emission spectrum of the smaller core size NAC-CdTeQDs (Fig. 2), which leads to better FRET between NAC-AuNPs and NAC-CdTeQDs. Secondly, the collision frequency between smaller NPs is higher than the larger ones. Furthermore, the PL enhancement of 2.4 nm (Fig. 5A) is higher than that of 1.7 nm NAC-AuNPs (Fig. 5B).

Fig. 5 PL enhancement of (A) 2.4 nm (0.10 mg mL⁻¹) and (B) 1.7 nm (0.10 mg mL⁻¹) NAC-AuNPs in 0.10 M PBS (pH 7.0) in the presence of various sizes of (green, yellow and red) NAC-CdTeQDs. The PL intensities were recorded at the λ_ex/λ_em of 400/813 nm and 400/780 nm for 2.4 nm and 1.7 nm NAC-AuNPs, respectively.

Luminescence quenching of NAC-CdTeQDs

It is well known that AuNPs can cause PL quenching of QDs. Fig. 6 depicts the PL of green NAC-CdTeQDs on the addition of 2.4 nm NAC-AuNPs at pH 6.0. By contrast to the PL enhancement of NAC-AuNPs, green NAC-CdTeQDs shows PL quenching when it interacts with NAC-AuNPs, which involves the FRET process from the NAC-CdTeQDs to NAC-AuNPs. The quenching of NAC-CdTeQDs does not arise from the inner filter effect of NAC-AuNPs which were proved by measuring the PL intensity of the separate NAC-CdTeQDs and NAC-AuNPs solutions as depicted in Fig. S2 (ESI†). Our results demonstrate that the PL quenching of NAC-CdTeQDs is mainly attributed to the FRET between NAC-AuNPs and NAC-CdTeQDs.

Fig. 6 PL spectra of green NAC-CdTeQDs (12.5 μM) in 0.10 M PBS (pH 6.0) in the presence of various concentrations (1) 0.00, (2) 0.25, (3) 0.50, (4) 1.00, (5) 1.50, (6) 2.00, and (7) 2.50 (μg mL⁻¹) of 2.4 nm NAC-AuNPs.

Fig. 7 shows the pH effect on the PL quenching of NAC-CdTeQDs by 2.4 nm NAC-AuNPs. All NAC-CdTeQDs display stronger PL quenching effect with the decrease of pH from 9.0 to 5.0. It is intelligible that the coulombic repulsion is the strongest at higher pH since most of the NAC-AuNPs and NAC-CdTeQDs are negatively charged (route 3 in Scheme 1). This will not favor the FRET-based interaction between NAC-CdTeQDs and NAC-AuNPs even though the overlap of absorption and emission spectra of NAC-AuNPs and NAC-CdTeQDs increases. Our results demonstrate that the intermolecular distance of NPs is very crucial for the PL quenching effect. In summary, neutral NAC-CdTeQDs is better quenched by NAC-AuNPs than their anionic counterparts. For example, NAC-CdTeQDs show very small quenching in the presence of NAC-AuNPs at pH 9.0. In addition, among the three sizes of NAC-CdTeQDs, green NAC-CdTeQDs display the highest PL quenching and follow by yellow and red NAC-CdTeQDs at pH 6.0. Under this situation, the overlap of absorption and emission spectra of NAC-AuNPs with the smallest NAC-CdTeQDs becomes more significant (Fig. 2), resulting in better FRET and presuming that the intermolecular distances between the NAC-AuNPs and NAC-CdTeQDs are the same for green, yellow and red NAC-CdTeQDs. However, when the pH increases to 7.0, PL quenching effect of green NAC-CdTeQDs decreases significantly and is lower than yellow and red NAC-CdTeQDs because of the more coulombic repulsion between the green NAC-CdTeQDs and NAC-AuNPs since their pK_a are 7.2 and 7.5–7.6 for the green and yellow/red NAC-CdTeQDs, respectively.

Fig. 7 Effect of pH on the PL quenching F_o/F of (A) green, (B) yellow and (C) red NAC-CdTeQDs by 2.4 nm NAC-AuNPs. The PL intensities were monitored at the corresponding λ_em of the NAC-CdTeQDs using λ_ex of 400 nm.

Fig. 8 depicts the effect of various sized NAC-AuNPs on the PL quenching of NAC-CdTeQDs at pH 7.0. The smaller size of NAC-AuNPs, the larger quenching on the NAC-CdTeQDs. 1.7 nm NAC-AuNPs displays the strongest quenching and follow by 2.4 nm and then 2.9 nm NAC-AuNPs. In the presence of 1.7 nm NAC-AuNPs, the PL quenching of yellow and red NAC-CdTeQDs is larger than green NAC-CdTeQDs, which is consistent with the trend of PL quenching in the presence of 2.4 nm NAC-AuNPs (vide supra). Finally, the PL quenching of three types of NAC-CdTeQDs by 2.9 nm NAC-AuNPs are similar, which is attributed to the small collision frequency of the largest core NAC-AuNPs.

Fig. 8 Effect of different sizes of 1.7 nm, 2.4 nm and 2.9 nm NAC-AuNPs on the PL quenching F_o/F of (A) green, (B) yellow and (C) red NAC-CdTeQDs in 0.10 M PBS (pH 7.0). The PL intensities were monitored at the corresponding λ_em of the NAC-CdTeQDs using λ_ex of 400 nm.

Conclusions

PL quenching and enhancement of different sized water-soluble NAC-CdTeQDs and NAC-AuNPs have been studied under various pH environments. The PL of NAC-CdTeQDs decrease in the visible region and the PL of NAC-AuNPs increase in the NIR region are consistent with FRET from the high-energy donor CdTeQDs to the low-energy acceptor NAC-AuNPs. In summary, direct interactions between water-soluble NAC-functionalized AuNPs and CdTeQDs were discussed by fluorescence changes of both AuNPs solution in NIR wavelength region and CdTeQDs in visible wavelength region, respectively. NIR fluorescence spectra of AuNPs were gradually enhanced with addition of CdTeQDs. Accordingly, a model was developed to describe the mechanism of different emission changes under different pH, where varying percentages of negative or neutral charged of AuNPs and CdTeQDs played an important role. Through detailed and intrinsic studies on spectroscopic analysis, illustrating that this system have potential applications for analysis detection and other purposes.

Experimental

Chemicals

Cadmium chloride (CdCl₂ > 99%), hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O > 99.9%) and tellurium powder (Te > 99%) were purchased from Aldrich (Milwaukee, WI). N-Acetyl-L-cysteine (NAC > 99%) and sodium borohydride (NaBH₄, 99%) were obtained from International Laboratory (San Bruno, CA). Acetone, ethanol (EtOH) and methanol (MeOH) were from Labscan (Bangkok, Thailand). Anhydrous sodium dihydrogen phosphate (NaH₂PO₄) and disodium hydrogen phosphate anhydrous (Na₂HPO₄) were purchased from Fluka (Buchs, Switzerland). Glacial acetic acid, hydrochloric acid (HCl) and sodium hydroxide (NaOH) were obtained from Farco Chemical Supplies (Beijing, China). Dialysis membrane tubes (MWCO = 10 000) were from Spectrum Laboratory (Rancho Dominguez, CA). Purified water from a Milli-Q-RO4 water purification system (Millipore, Bedford, MA) with a resistivity higher than 18 MΩ cm⁻¹ was used throughout. All reagents of analytical grade were used without further purification.

Synthesis of water-soluble N-acetyl-L-cysteine-protected AuNPs

The water-soluble AuNPs were synthesized and purified according to a previous method,^40,41 in which NAC was selected to protect Au core by means of the thiol coordination. Size of AuNPs used in this work was controlled by different Au/NAC mole ratios. Briefly, to a 200 mL solvent mixture of MeOH/acetic acid (6 : 1 v/v), HAuCl₄·3H₂O (1.82 g, 4.62 mM) and different amounts of NAC with various Au/NAC mole ratios (1 : 3, 1 : 1 and 3 : 1) were added under stirring at 0 °C to form a white suspension of orange solution. The mixture was then reduced with 3.50 g (92.5 mM) of NaBH₄ in 45 mL of ethanol, turning to a dark brown solution immediately. After stirring for 30 min, the NAC-AuNPs product was precipitated by adding 200 mL of acetone, then adjusted pH to 1.0 by concentrated HCl, washed with acetone, and further purified by dialysis for 7 days. Finally, the dark brown NAC-AuNPs solution, which was collected from the dialysis membrane tubes, was dried by a stream of N₂ at room temperature.

Synthesis of water-soluble N-acetyl-L-cysteine-stabilized CdTeQDs

Fluorescent NAC-CdTeQDs were prepared based on a previously described⁴⁴ method with slight modifications. First, 40 mg of NaBH₄ was added to react with 64 mg of Te powder in 2.0 mL of water to produce sodium hydrogen telluride (NaHTe). This course lasted for approximately 5 h in a round-bottomed flask stirring at 0 °C. Then, freshly prepared NaHTe aqueous solution was injected to a series of 1.25 mM CdCl₂ solutions at pH 10.0 in the presence of NAC acting as a stabilizing agent. The molar ratio of Cd²⁺/NAC/HTe⁻ was fixed at 1 : 2.4 : 0.5. The reaction went along with N₂ protection. After stirring for 5 min at room temperature, the mixtures were transferred to a reflux and continued to react at 200 °C for 30, 40 and 50 min, resulting in three different sizes of NAC-CdTeQDs, respectively. Finally, the reaction mixtures were cooled to room temperature.

Absorption and photoluminescence spectroscopy

Absorption spectra of the NAC-AuNPs and NAC-CdTeQDs solutions were taken on a Varian Cary 100 Scan UV/visible absorption spectrophotometer (Palo Alto, CA). Corrected PL spectra were recorded on a Photon Technology International QM4 spectrofluorometer equipped with a photomultiplier (PMT) detector and a thermoelectrically cooled InGaAs photodiode detector for visible and NIR regions measurement, respectively (Lawrenceville, NJ). All the NAC-AuNPs and NAC-CdTeQDs solutions were prepared in 0.10 M phosphate buffer solutions (PBS) at various pH values. To investigate the luminescence quenching of NAC-CdTeQDs by NAC-AuNPs, a 2.0 mL aliquot of 12.5 μM NAC-CdTeQDs solution was placed in a standard 10 mm path-length quartz cuvette. Then various volumes (0.0–20 μL) of 0.5 mg mL⁻¹ NAC-AuNPs solution were added. Excitation and emission bandwidths were set at 10 and 8.0 nm respectively to record the changes in luminescence intensity of NAC-CdTeQDs by the PMT detector. To study the PL enhancement of NAC-AuNPs by NAC-CdTeQDs, a 2.0 mL aliquot of 0.1 mg mL⁻¹ NAC-AuNPs solution was transferred into the 10 mm path-length quartz cuvette. Various volumes (0.0–20 μL) of 1.25 mM NAC-CdTeQDs solution were added. Excitation and emission bandwidths were set at 10 and 12 nm respectively for the changes in NIR intensity of NAC-AuNPs using the InGaAs photodiode detector. A 555 nm long-pass color filter (Corion, Holliston, MA) was placed between the cuvette and detector to remove the scattering and excitation light interference. All NAC-CdTeQDs and NAC-AuNPs samples were excited at 400 nm and optical measurements were carried out at ambient conditions.

Acknowledgements

This work was supported by the National Science Foundation of China (21575084 and 21475080) and the Hundred Talent Programme of Shanxi Province.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Absorption of AuNPs, CdTeQDs, mixture of AuNPs and CdTeQDs, and the mixture substraction AuNPs, fluorescence spectra of CdTeQDs and PBS, CdTe and AuNPs in PBS, and mixture (CdTeQDs and AuNPs) and AuNPs. See DOI: 10.1039/c6ra18456a

‡ Present address: Acadia Divinity College, Acadia University, 15 University Avenue, Wolfville, Nova Scotia, B4P 2R6, Canada.

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