Thiol-Ligand-Catalyzed Quenching and Etching in Mixtures of

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Thiol-Ligand-Catalyzed Quenching and Etching in Mixtures of Colloidal Quantum Dots and Silver Nanoparticles Jae-Seung Lee,*,† Hyungki Kim,‡ and W. Russ Algar*,‡ †

Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841 Republic of Korea Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada

‡

S Supporting Information *

ABSTRACT: Plasmonic nanostructures have the potential to enhance the emissive properties of semiconductor quantum dots (QDs). Although gold nanoparticles have been widely used for this purpose, other metals, such as silver, are also of interest and have more desirable plasmonic properties; however, silver nanoparticles suffer from chemical instability that gold nanoparticles do not. We find that this instability has the potential to limit the integration of silver nanoparticles (AgNPs) with QDs. Specifically, the common selection of thiol ligands for colloidal stabilization of QDs is incompatible with AgNPs, whether silver nanospheres or silver nanoplates. Equilibrium desorption of thiol ligands from QDs drives a pseudocatalytic process wherein the AgNPs are etched to produce silver(I)−ligand complexes, which then undergo cation exchange reactions at the QD leading to quenching of its photoluminescence (PL) through the introduction of long-lived trap states. We characterize this process through a combination of morphological, chemical, and steady-state and time-resolved spectroscopic measurements. The latter include extinction and absorption, PL emission intensity and lifetime, and transient absorption. Importantly, the etching and quenching process is avoided with QDs that are coated with an amphiphilic polymer instead of thiol ligands.

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INTRODUCTION The use of plasmonics to enhance or otherwise modify the emissive properties of luminescent materials has been a major subject of research in photonics.1−5 Metallic nanostructures are the crux of this research. Large enhancements in emission can be observed when a luminescent material is placed in close proximity to, but not in contact with, a metallic nanostructure.1,2 In other cases, metallic nanostructures are efficient quenchers of emission6−9 or mediate and enhance nonradiative energy transfer between a luminescent donor material and an acceptor material.10−14 The foregoing processes are generally maximized when the absorption or emission spectrum of the luminescent material overlaps with the resonance of a localized surface plasmon (LSP) supported by the metallic nanostructure. Plasmonic modification of emission from luminescent materials has practical applications in chemical and biological detection schemes and signal amplification, facilitating singlemolecule spectroscopy and engineering enhanced optoelectronic devices.1−5,15−17 For success in these applications, metal nanostructures and luminescent materials must be effectively integrated, which is the challenge we address in this study. Among the many luminescent materials that can benefit from integration with plasmonic nanostructures, colloidal semiconductor quantum dots (QDs) are particularly appealing because of their generally superior optical properties. These properties include but are not limited to bright, spectrally © XXXX American Chemical Society

narrow, tunable, and robust photoluminescence (PL) emission and the ability to engage in electron transfer and nonradiative energy transfer (e.g., Förster type).18,19 The current and envisioned applications of QDs are also a good match to those noted above for plasmonics. There are many examples of combinations of QDs and colloidal metal nanoparticles with interesting photonic behavior: enhancement of QD PL intensity up to an order of magnitude,20,21 enhancement of energy transfer rates between QDs,11 and quenching of QD PL over long distances,8,22−25 to name but a few. Although gold nanoparticles are the most common material utilized for these purposes, silver nanoparticles are also of interest. Silver has a stronger and sharper LSP resonance than gold, and the resonance is shifted to shorter wavelengths (400−450 nm for silver vs 500−600 nm for gold), making it generally more suitable for matching excitation wavelengths for metal-enhanced emission.26−28 A challenge, however, is that silver nanoparticles are prone to oxidation, etching, and other chemical reactions.29−31 Their effective use in plasmonic architectures is dependent on the formation of stable assemblies with luminescent materials such Received: October 19, 2017 Revised: November 22, 2017

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DOI: 10.1021/acs.jpcc.7b10381 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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0.2 mL of aqueous poly(acrylic acid) (PAA; MW ∼ 1800 Da) solution (100 g/L). The mixture was incubated at 25 °C for 2 h. The resulting PAA-AgNPLs were centrifuged at 8600 rcf for 10 min. The supernatant was removed, and the pellet was redispersed in borate buffer (pH 9.5, 100 mM). PAA-AgNPLs were then pelleted, washed, and redispersed with borate buffer three more times and stored at 4 °C until needed (1.0 OD at 630 nm). The PAA stabilized the AgNPL under buffer conditions for more than 50 h (see Supporting Information, Figure S1). Analogous procedures were used for PAA-AgNS and PAA-AuNS, which had 2 nM concentrations before and after modification. X-QDs, where X was glutathione (GSH), dihydrolipoic acid (DHLA), or mercaptopropionic acid (MPA), were prepared using published ligand exchange methods.34,35 Polyethylene glycol-appended dihydrolipoic acid (DHLA-PEG; average MW of PEG ∼ 750 Da) was synthesized and ligand exchanged onto QDs as reported previously.36,37 Quenching Experiments. In a typical experiment, PAAmetal nanoparticles in borate buffer (100 mM, pH 9.5; 50 μL) were combined with X-QDs in borate buffer (50 μL) at 25 °C in a nonbinding 96-well plate. Concentrations were varied between 0 and 200 nM for QDs and 0.1X and 5X for metal nanoparticles. Extinction and PL measurements were made at regular intervals after mixing. PL decay and transient absorption measurements were made after 14 h.

as QDs, but achieving such stability with silver nanoparticles is far less trivial than it is with gold nanoparticles. Here, we identify and characterize a critical limitation with respect to the assembly of silver nanoparticles and QDs. The limitation pertains to systems where the silver nanoparticles are colloidally stabilized with ligands such as citrate and poly(acrylic acid) and where the QDs are colloidally stabilized with ligands that have thiol anchoring groups. Such coatings are very common, if not the most common for both materials, making this limitation significant. We find that the equilibrium desorption of thiol ligands from QDs leads to etching of silver nanoparticles, the formation of silver−thiol ligand complexes, and quenching of QD PL through trap-inducing cation exchange reactions. The successful integration of silver nanoparticles and QDs into plasmonic assemblies therefore requires that either the QDs or silver nanoparticles are coated with chemistry that avoids or blocks this process.

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EXPERIMENTAL SECTION Materials. Gold nanospheres (AuNS; 15 nm diameter) and alloyed CdSeS/ZnS QDs were from Cytodiagnostics (Burlington, ON, Canada). CdSe/CdS/ZnS QDs were synthesized as reported previously.32,33 All QDs are denoted as X-QDλ(a), where X is the coating that disperses the QD in aqueous buffer and λ(a) is the wavelength of peak PL emission and (a) indicates an alloyed core (when applicable). Silver nanospheres (AgNS; 15 nm diameter) were from Ted Pella (Redding, CA), and silver nanoplates (AgNPL) were synthesized as reported previously.29 Solutions of metal nanoparticles with an extinction of 0.5 OD at λ MAX were denoted as 1X concentration. Instrumentation. Extinction and PL emission spectra were measured using an Infinite M1000 multifunction plate reader (Tecan Group Ltd., Morrisville, NC). Emission spectra were recorded with 230 nm excitation, a 2 nm step size, and 5 nm monochromator bandwidths. Time-resolved PL decays and transient absorption (TA) spectra were measured using a picosecond laser and streak camera system. For PL decay measurements, laser pulses at 266 nm were from the fourth harmonic of a high energy picosecond mode-locked Nd:YAG laser (PL2241; EKSPLA, Vilnius, Lithuania) with a 10 Hz repetition rate. For TA measurements, a pump pulse at 430 nm was generated by a picosecond optical parametric generator (PG401; EKSPLA, Vilnius, Lithuania) coupled with the Nd:YAG laser. The probe was at 90° to the pump and was from a xenon breakdown cell pumped by residual 1064 nm output from the Nd:YAG laser. Time-resolved PL decay spectra and TA spectra were recorded with an imaging monochromator (Spectra Pro 2300i; Princeton Instruments, Trenton, NJ) and streak camera (C7700; Hamamatsu Photonics, Hamamatsu, SZK, Japan). Measurements of PL emission lifetimes were made in photon counting mode. For TA measurements, a 560 nm long-pass filter was placed prior to the monochromator. Transmission electron microscopy (TEM) images were obtained using a Tecnai 20 (FEI-Thermo Fisher Scientific, Hillsboro, OR) operated at 200 kV. Electrospray ionization (ESI)-mass spectra were obtained using a TSQ Quantum Ultra EMR triple quadrupole mass spectrometer (Thermo Fisher Scientific, Waltham, MA). X-ray photoelectron spectra were measured using a Leybold MAX200 spectrometer (Leybold, Germany) with an unmonochromated Mg Kα source (192 eV). Surface Modification of Nanoparticles. An aqueous solution of as-synthesized AgNPL (1.8 mL) was combined with

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RESULTS AND DISCUSSION Materials and Optical Properties. Figure 1 shows representative examples of extinction and, if applicable, PL emission spectra for the materials in this study. The absorption spectrum of each QD material exhibited a defined first-exciton

Figure 1. (A) UV−visible extinction spectra of AgNS, AuNS, and AgNPL. (B) UV−visible absorption spectra (solid line) and PL spectra (dotted line) of QD525a and QD545. B

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The Journal of Physical Chemistry C peak, and its PL emission spectrum exhibited a single, spectrally narrow, and symmetric peak (fwhm ∼30 nm). These spectral features corresponded to excitation and recombination of an exciton across the quantum-confined conduction and valence band edge states of the QD. The extinction spectra of the AgNPL, AgNS, and AuNS exhibited strong bands at 630, 410, and 520 nm, respectively, corresponding to excitation of a sizedependent plasmon. Anomalous Quenching of QD PL. As alluded to earlier, the motivation for pairing QDs and AgNPL was to create assemblies with plasmon-enhanced emission. Instead, it was found that mixing AgNPL and GSH-QD525a in solution led to a progressive decrease in the band-edge PL of the QD over time. This trend, shown in Figure 2A, was accompanied by a small but progressive increase in PL emission intensity at ∼650 nm (see inset), consistent with band-gap PL emission. Quenching of the band-edge PL emission thus appeared to be linked to the formation of trap states within the band gap, somehow induced by the AgNPL. To better characterize the quenching of QD PL upon mixing with AgNPL, the emission intensity of the GSH-QD525a at ∼525 nm (band-edge) and ∼650 nm (band-gap) was measured as a function of time for different concentrations of QD with a fixed concentration of AgNPL. As shown in Figure 2B, bandgap emission was not prominent at the lowest concentrations of QD (≤2 nM) but was more clearly observed at higher concentrations. The rate of appearance of band-gap emission was slower at higher concentrations of QD, although the maximum intensity of the band-gap emission appeared to scale proportionally with the QD concentration. The band-edge emission also decreased more slowly at higher concentrations of QD. Taken together, these results suggested a cumulative quenching process, where a minimum amount of a AgNPLderived quenching agent was required to induce band-gap PL. Sufficient accumulation of this agent and putative trap states appeared to quench not only the band-edge PL but also eventually the band-gap PL. The band-gap PL is presumably quenched as the trap states become more numerous or energetically deeper. Next, the converse experiment was done with a fixed concentration of GSH-QD525a and different concentrations of AgNPL. The PL intensities at ∼525 nm (band-edge) and ∼650 nm (band-gap) were again tracked over time after mixing the QDs and AgNPL. Figure 2C shows that the band-gap PL intensity increased and decayed more rapidly at higher concentrations of AgNPL; however, the maximum band-gap PL intensity of the GSH-QD was approximately the same regardless of AgNPL concentration. These trends were consistent with the idea of a cumulative effect from an AgNPL-derived quenching agent, where higher concentrations of AgNPL increased the rate of the quenching process but did not change its end point. Concurrent Etching of AgNPL. Given the changes in QD PL after mixing with AgNPL, extinction spectroscopy was used to determine if there were reciprocal effects on the AgNPL. Figure 3A shows that the extinction of the AgNPL progressively decreased in intensity and blue-shifted from ∼630 nm to ∼600 nm over 14 h after mixing with GSH-QD525a. Figure 3B shows the results from mixing AgNPL with a series of concentrations of GSH-QD525a for 18 h. The extinction of the AgNPL decreased as the concentration of QDs increased. A parallel blue-shift was observed up to a point (∼100 nM), after which there was a gradual red-shift. These spectral changes implied

Figure 2. (A) PL spectra of the GSH-QD525a (100 nM) were obtained every hour in the presence of the AgNPL (0.5 OD; 1X). The bandedge emission at 525 nm decreased as the band-gap emission at 650 nm increased (bottom inset). The PL colors from the GSH-QD525a at different incubation times are also shown (top inset). (B) Timedependent changes in the band-edge and band-gap PL emission intensity of the GSH-QD525a at QD concentrations between 0 and 200 nM. (C) Time-dependent changes in the band-edge and band-gap PL emission of the GSH-QD525a for AgNPL concentrations between 0.1X and 5X.

that the AgNPLs were etched in proportion to the concentration of QDs and that the AgNPL destabilized and aggregated with more extensive etching. As expected, the rate and extent of etching increased as temperature increased (see Figure S2). Quenching and Etching with Other Materials. To determine the scope of the foregoing results, both the QD and metal nanoparticle materials were varied. The surface ligand coating on the QDs was varied, and the experiments in Figure 2 were repeated with X-QD525a, where X was DHLA, MPA, and DHLA-PEG. Upon mixing with AgNPL, C

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Figure 4. (A) PL spectra of GSH-QD525a (100 nM) in the presence of AuNS (1X) as a function of time. Unlike the AgNPL and AgNS, the AuNS had little or no effect on the band-edge and band-gap PL emission (inset). (B) There were negligible changes in the UV−visible extinction spectrum of the AuNS in the presence of GSH-QD525a (200 nM).

Figure 3. (A) UV−visible extinction spectra of AgNPL in the presence of GSH-QD525a (100 nM) as a function of time. As the reaction progressed over 14 h, the wavelength blue-shifted from 630 to 600 nm. (B) Extinction spectra of the AgNPL in the presence of GSH-QD525a at concentrations between 0 and 2 μM. Note that the AgNPLs exhibit various colors as a result of etching and aggregation (inset photo).

Identifying the Quenching Agents. Given the results thus far, we hypothesized that Ag+ ions originating from the AgNPL and AgNS created surface trap states on the QDs. These trap states then caused quenching of band-edge PL and evolution of band-gap PL. Many metal ions have been reported to quench band-edge QD PL, including Ag+ ions.38−41 A second aspect of our hypothesis was a role for the thiol ligands bound to the QDs. These ligands are in dynamic equilibrium, and some desorb from the QD upon dilution.42,43 Free thiol ligands are capable of etching the silver NPs,44−46 presumably resulting in the formation of silver(I)−thiol ligand complexes.47−49 To test our hypothesis, we reacted AgNPL with free GSH ligands (no QDs) at three concentrations, pelleted the AgNPL, and collected only the supernatant containing the products of the putative reaction between the AgNPL and GSH. When this supernatant was combined with GSH-QD525a, band-gap PL appeared almost immediately with an intensity that scaled with the concentration of free GSH ligand and increased over time, as shown in Figure 5A. Analogous results were obtained when the experiment was repeated with AgNS instead of AgNPL (see Figure S7). The quenching agents derived from the silver nanoparticles thus seemed likely to be silver(I)−GSH complexes.

the DHLA-QD525a exhibited trends in band-edge PL, band-gap PL, and etching of AgNPL that mirrored those with GSHQD525a, albeit with slower rates of change in the band-gap PL (see Figure S3). MPA-QD525a and PEG-QD525a were also quenched by AgNPL with evolution of band-gap PL, although a clear trend in the maximum band-gap PL intensity was not seen (see Figures S4 and S5). The same overall quenching and etching behavior was thus observed with different thiol ligands on the QD. Next, the AgNPLs were replaced with AgNS and AuNS. Upon mixing with the AgNS, the GSH-QD525a exhibited changes in band-edge and band-gap PL that were largely analogous to the results with AgNPL, and etching of the AgNS was evident from time-dependent changes in their extinction spectra (see Figure S6). However, when the AgNSs were replaced with AuNS, quenching of GSH-QD525a band-edge PL was negligible, and no significant band-gap PL appeared. There was also no indication of etching of the AuNS, as shown in Figure 4. These cumulative results suggested that the observed quenching of QD PL was associated with the silver component of the AgNPL and AgNS and was not special to a specific shape of silver nanoparticle nor a general phenomenon with all metal nanoparticles. D

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Figure 5B shows that all PL was rapidly quenched with 100 μM and 1 mM silver(I)−GSH complexes, but evolution of bandgap PL was observed with 10 μM and 1 μM complexes. These silver(I)−GSH complexes were confirmed to quench X-QD525a with ligands other than GSH and also quench other QD nanocrystals, including QD540a, QD545, QD605, QD630, and QD650 (see Figure S8). The supernatant from the mixture of AgNPL and free GSH ligands was analyzed by ESI mass spectrometry (ESI-MS) and compared to the silver(I)−GSH complexes deliberately synthesized by mixing AgNO3(aq) and free GSH ligand. As seen in Figure 5C, the mass spectra (negative ion mode) had groups of peaks centered at ca. 826 and 933 (m/z), consistent with [2Ag+ + 2GSH−3H]− and [3Ag + + 2GSH−4H] − complexes, respectively. This correspondence confirmed that Ag+−thiol ligand complexes were the agents that quenched the QD PL. Cation Exchange. With the quenching agent identified, we sought to confirm that the Ag+−GSH complexes (or Ag+−thiol ligand complexes, more generally) quenched QD PL through cation exchange reactions that formed trap states within the band gap. Morphological and compositional changes in the GSH-QD525a after the putative cation exchange reaction and purification were investigated via TEM. Figure 6A shows that

Figure 6. (A) TEM images of the GSH-QD525a that were (I) not reacted and reacted with silver(I)−GSH complexes at (II) 10 μM and (III) 20 μM. (B) XPS spectra of the GSH-QD545 that were unreacted (black) and reacted with silver(I)−GSH complexes (red). A new peak at ∼367 eV appeared after the reaction (indicated by the green arrow), indicative of the presence of Ag+ (3d5/2) in the QDs after the cation exchange reaction. Figure 5. (A) Time-dependent changes in the band-gap PL emission intensity (650 nm; left ordinate, solid line) and band-edge PL emission intensity (525 nm; right ordinate, dotted line) of GSH-QD525a when mixed with the supernatant from the reaction between AgNPL and GSH at 1, 10, and 100 μM. (B) Data analogous to panel A for GSHQD525a mixed with synthesized silver(I)−GSH complexes ([Ag+] = 1, 10, 100, 1000 μM with [Ag+]:[GSH] = 1:2). (C) ESI mass spectra (negative ion mode) for synthesized silver(I)−GSH complexes (black) and for the supernatant from a mixture of AgNPL and GSH (red).

the size and shape of the QDs were unchanged after incubation with Ag+−GSH complexes. The atomic percentage of the silver in the QDs was concurrently analyzed by energy-dispersive Xray spectroscopy (EDX), and values of ∼1.0% were obtained. The introduction of silver into the QDs was also confirmed via X-ray photoelectron spectroscopy (XPS). Figure 6B shows that unreacted GSH-QD545 did not exhibit any XPS peaks that corresponded to a binding energy for silver, whereas GSHQD545 that were reacted with Ag+−GSH complexes had a clear peak at ∼367 eV for Ag 3d5/2.50 After normalization to exclude background carbon, oxygen, and silicon, the atomic percentage

For further confirmation, silver(I)−GSH complexes were deliberately synthesized at four concentrations by mixing AgNO3(aq) with GSH and then added to the GSH-QD525a. E

DOI: 10.1021/acs.jpcc.7b10381 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C of silver in the QDs was determined to be 4.7%. The combined TEM and XPS data confirmed that the Ag+−GSH complexes underwent cation exchange reactions with the QDs. Trap States. Confirmation that the products of cation exchange were trap states was sought in time-resolved spectroscopic measurements. First, the decay rates of the band-edge and band-gap PL emission of GSH-QD525a were measured before and after mixing with AgNPL. Figure 7 shows

Figure 8. Transient absorption spectra (left) and decay curves corresponding to the 700−750 nm spectral region (right) for (A) the original GSH-QD545, (B) GSH-QD545 reacted with the AgNPLs, and (C) GSH-QD545 reacted with synthesized silver(I)−GSH complexes. Figure 7. Normalized PL decay curves for (A) GSH-QD525a, measured at 525 nm, and (B) GSH-QD525a, reacted with AgNPLs, measured at 525 nm (black curve) and 650 nm (red curve).

excitonic states within the band gap after initial photoexcitation of the QD. Moreover, the only partial decay of the TA feature between 700 and 750 nm after 90 ns was consistent with the observation of long-lived band-gap PL and confirmed that the band-gap states were traps. Quenching Mechanism. Figure 9 shows our proposed mechanism for quenching of QD PL when these materials are

that the GSH-QD525a exhibited only band-edge PL with a decay curve that fit to a biexponential model with an amplitudeweighted average lifetime of 11.6 ns. After mixing with AgNPL, the GSH-QD525a had amplitude-weighted lifetimes of 189 and 279 ns for the band-edge PL at ∼525 nm and emergent bandgap PL at ∼650 nm, respectively. These order of magnitude decreases in decay rate after mixing with AgNPL were indicative of the formation of trap states. Next, transient absorption (TA) spectra were measured for GSH-QD545 before and after mixing with both AgNPL and deliberately synthesized Ag+−GSH complexes. The GSHQD545 exhibited the same quenching of band-edge PL and emergence of band-gap PL that was observed with GSH-QD525a (see Figure S8C). The most informative region of the TA spectrum was wavelengths longer than 600 nm. Figure 8A shows that the original GSH-QD545 had virtually no transient absorption at these wavelengths. After reaction between the GSH-QD545 and AgNPL, the rise and partial decay of a broad feature at wavelengths longer than 600 nm was observed, as per Figure 8B. The same feature was observed after reaction with deliberately synthesized Ag+−GSH complexes, as shown in Figure 8C. Both results were consistent with population of

Figure 9. Proposed mechanism for gradual quenching of QD PL and etching of silver nanoparticles upon mixing. Thiol ligands that desorb from the QD have a catalytic role. F

DOI: 10.1021/acs.jpcc.7b10381 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C coated with thiol ligands and mixed with silver nanoparticles. Thiol ligands desorb from the QD and etch the silver nanoparticle to form silver(I)−ligand complexes. These complexes transport Ag+ ions to the QDs where the cation exchange reaction occurs, introducing trap states within the band gap. The result is gradual etching of the silver nanoparticle and quenching of QD PL. The latter includes quenching of band-edge PL, initial evolution of band-gap PL, and subsequent quenching of band-gap PL as the trap states become more numerous or deeper. The ligand has a catalytic role in this process because it cycles between its free form, complexes with Ag+ ion(s), and its QD-bound form, although it is unclear if specific ligand molecules are actually recycled. The data in this study account for each step in the proposed mechanism: silver(I)−thiol ligand complexes were indeed observed to form when free ligand was mixed with silver nanoparticles; the silver nanoparticles were etched; silver was observed in the composition of the QD nanocrystals after mixing with silver(I)−thiol ligand complexes; the effect of silver(I)−thiol ligand complexes on QDs was the same as the effect of silver nanoparticles; and trap states were confirmed with spectroscopic measurements. We can also exclude the possibility of unpurified excess thiol ligands from ligand exchange of the QDs driving the process because increasing the number of purification cycles following ligand exchange did not diminish the quenching effect. The top half of the mechanism in Figure 9, with migration of the ligand from QD to metal nanoparticle, may also occur with metals other than silver, for example, gold, with which thiols also bind strongly. However, gold is not readily etched by thiols, so the mechanism stops at this point and the migration cannot be observed by the same methods that revealed the mechanism with silver nanoparticles. Other metals, such as copper, are anticipated to behave more like silver. Evading Quenching. The mechanism of quenching in Figure 9 implies that a potential approach for successfully using silver nanoparticles with QDs is functionalization of the latter with a coating that is not based on thiol ligands. Amphiphilic polymers (APs) are a good candidate coating for this purpose. Instead of replacing the hydrophobic surfactant ligands from the solvothermal synthesis of QDs, as GSH, DHLA, and other thiol ligands do, an AP coating has pendant alkyl chains that interdigitate with the hydrophobic surfactant ligands and wrap around the as-synthesized QD. Hydrophilic carboxylate groups remain exposed for aqueous dispersion of the nanocrystals. To test if quenching could be avoided, AP-QD520 and GSHQD520 were mixed with AgNS coated with either citrate or PAA, and their PL was tracked over time. Figure 10 shows that the band-edge PL intensity (at ∼525 nm) for the GSH-QD520 gradually decreased, while the band-gap PL intensity (at ∼780 nm) increased initially and then slowly quenched at longer times after mixing with citrate-AgNS. A much slower but steady increase in band-gap PL was observed after mixing GSH-QD520 with PAA-AgNS, accompanying slower quenching of the bandedge PL. In contrast, Figure 10 also shows that no band-gap PL was observed when AP-QD520 was mixed with AgNS, and only a small and ultimately stable decrease in band-edge PL intensity was observed. These trends were in agreement with expectations from the proposed quenching mechanism. Moreover, the different rates of quenching between PAA-AgNS and citrateAgNS paralleled the expected affinity of each coating for the AgNS, suggesting that more robust coatings on silver

Figure 10. Time-dependent changes in the relative band-gap (BG; open circles) and band-edge (BE; solid circles) PL intensities for YAgNS after mixing with (A) GSH-QD520 and (B) AP-QD520, where Y = citrate (Cit) or PAA.

nanoparticles may also be useful in minimizing etching and quenching in mixtures with QDs. Discussion. The new knowledge from this study is neither the observation that thiol ligands can etch silver nanoparticles nor the observation that silver ions undergo cation exchange reactions with QDs. The etching of AgNPL by thiol compounds has been studied previously,45 and cation exchange reactions with silver ions have been purposefully used in synthetic contexts with colloidal QDs.51−55 Rather, the new knowledge is that the catalytic (or pseudocatalytic) process depicted in Figure 9 is sufficient to drive these processes in a mixture of silver nanoparticles and QDs stabilized by thiol ligands. The processes need not be driven by the addition of large quantities of a thiol reagent to silver nanoparticles or by the addition of large quantities of Ag+ ion to solutions of QDs. Equilibrium desorption of the thiol ligand from the QD is sufficient. Consequently, QDs should be colloidally stabilized with an amphiphilic polymer or another functionally equivalent chemistry if intended to be mixed with silver nanoparticles, for example, as part of a plasmonic system. Our results also call for reassessment of previous studies that have reported plasmon-enhanced band-gap emission from QDs combined with silver nanoparticles.56−60 Many, although perhaps not all, of the observations in these previous studies are also consistent with the etching and quenching mechanism we have elucidated here. Some ambiguity arises with these studies because of the lack of characterization of the silver nanoparticles after mixing with QDs and because the QDs had intrinsic trap states and band-gap PL emission after synthesis, prior to mixing with silver nanoparticles. Increases in band-gap PL emission intensity thus could have been from a genuine plasmonic enhancement, from the formation of additional trap states through a mechanism similar to Figure 9, or from a combination of these two processes. G

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CONCLUSIONS We investigated the quenching of band-edge PL emission and the emergence of band-gap PL emission upon mixing colloidal QDs stabilized by thiol ligands with silver nanoparticles. A combination of spectroscopic and other physicochemical measurements provided insight into the mechanism of quenching. Thiol ligands desorb from the QDs and etch the silver nanoparticle to form silver−thiol ligand complexes, which then undergo cation exchange reactions with the QD, forming trap states that account for the trends in band-edge and bandgap PL emission. This behavior was consistent across multiple QD and silver nanoparticle materials but was not observed when QDs were mixed with gold nanoparticles, nor when the thiol ligands on the QD were replaced with an amphiphilic polymer coating. We have thus elucidated an important limitation to heed when working with silver nanoparticles and QDs. These results will help enable new advances in using silver nanoparticles for plasmonic enhancement or modification of QD PL.

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(LASIR) at UBC. XPS measurements were done at the Interfacial Analysis & Reactivity Laboratory (IARL) at UBC.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10381. Additional results including time-dependent UV−vis spectra of PAA-AgNPL, time-dependent extinction changes of AgNPL at various temperatures, timedependent PL changes of QDs coated with various thiol ligands, time-dependent spectral changes of AgNS and QDs, and time-dependent PL spectra of various QDs in the presence of silver(I)−GSH complexes (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.S.L.). *E-mail: [email protected] (W.R.A.). ORCID

Jae-Seung Lee: 0000-0002-4077-2043 W. Russ Algar: 0000-0003-3442-7072 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS W.R.A. and H.K. acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI). W.R.A. is grateful for a Canada Research Chair (Tier 2), a Michael Smith Foundation for Health Research Scholar Award, and an Alfred P. Sloan Fellowship. H.K. is grateful for a NSERC Canada Graduate Scholarship and a UBC 4YF award. J.S.L. acknowledges support from the NRF funded by the Korean government, MSIP (NRF-2015M3A9D7031015, NRF2016R1A5A1010148, and NRF-2015R1C1A1A01053865). The authors appreciate helpful discussion with Dan Bizzotto and thank Saeid Kamal, Melissa Massey, Eleonora Petryayeva, and Ken Wong for their experimental assistance. The ESI-MS spectra were obtained using the facilities of the Korea Basic Science Institute (KBSI; Seoul, Republic of Korea). Transient absorption and PL decay measurements were done in the Laboratory for Advanced Spectroscopy and Imaging Research H

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