Effect of Protons on CdSe and CdSe−ZnS Nanocrystals in OrganicSolutionTommaso Avellini, Matteo Amelia, Alberto Credi, and Serena Silvi*
Dipartimento di Chimica “G. Ciamician”, Universita degli Studi di Bologna, via Selmi 2, 40126 Bologna, Italy
*S Supporting Information
ABSTRACT: Core and core−shell quantum dots are covered with alayer of organic ligands which prevents aggregation and eliminates surfacedefects, thus enhancing the photophysical properties and stability of thematerial. These ligands are usually Lewis bases and can therefore beaffected by the presence of acid in the surrounding environment. Wesynthesized core CdSe and core−shell CdSe−ZnS quantum dots withvarious shell thicknesses and different organic ligands, and weinvestigated the effect of acid and base on their photophysical properties.In dilute CHCl3 solution, the organic ligands can be protonated uponaddition of acid and detached from the surface of the nanoparticles. As aconsequence, the nanoparticles aggregate and their luminescence isquenched. Aggregated particles can be partly disgregated and theluminescence restored by deprotonation of the free ligands with a base.Since the presence of organic ligands on the surface is an essential characteristic of quantum dots, these effects should be takeninto consideration when designing quantum dot-based sensors.
■ INTRODUCTION
Colloidal semiconductor nanocrystals, also known as quantumdots (QDs),1−3 have generated great interest in recent years forboth fundamental science1,4−7 and applications2,3 related tobiological sensing and imaging,8−10 optoelectronics,11−13 andsolar energy conversion.14,15 The main features that render theQDs appealing substitutes of organic dyes for luminescenceapplications16 are the broad absorption profile and the intense,narrow, and size-dependent emission coupled to a highphotobleaching resistance and versatile surface functionaliza-tion.The achievement of valuable and reproducible spectroscopic
properties (narrow and size-tunable emission, high lumines-cence quantum yields, long-term photostability) is, however,strictly related to the quality of the material, particularly as faras surface states are concerned.17 This issue is a directconsequence of the small dimensions of the QDs andparticularly of the fact that the surface area-to-volume ratioincreases as the particle diameter decreases; hence, for smallQDs, a substantial fraction of the total atoms are on the surface.Such atoms constitute defects in the nanocrystal structurebecause of the presence of unsaturated valences and danglingbonds;18 moreover, they are exposed to attack of externalagents (e.g., oxygen and water). The presence of defectsprevents radiative recombination of the charge carriers byenabling nonradiative decay pathways that involve trapping ofthe electron and/or hole in the defects.19 Surface states affectsubstantially the redox properties of semiconductor QDs20 andare also believed to play a role in other basic phenomena suchas photoblinking21,22 and photobrightening.23
Understanding the response of semiconductor QDs toenvironmental changes is of the highest importance for atleast two reasons. On the one hand, a knowledge of the rangeof experimental conditions in which these nanomaterials arestable enables their manipulation and detailed investigation. Onthe other hand, the intrinsic sensitivity of the QDs to a specificanalyte can be exploited for chemo(bio)sensing applications,possibly without further chemical functionalization of thesurface.24,25
One of the most environmentally and analytically relevantparameters in liquid solutions is certainly proton concentration.The influence of pH on the spectroscopic properties of water-soluble core and core−shell nanocrystals was addressed inseveral investigations.26−35 Although a quenching of the QDluminescence is usually observed in acidic environment,36,37
some studies have reported an enhancement of theluminescence of CdTe nanocrystals on decreasing the pH ofthe solution.38−41 The reversibility of the effects of the acidaddition on the photophysical properties of the QDs was alsoinvestigated.34,35,39,40 For example, Peng and co-workers30
carried out a detailed study on the effect of pH on thiol-coatedcadmium chalcogenide core nanocrystals in aqueous solutionand found that the acid competes with the QDs for the ligands,which are Lewis bases. Therefore, at low pH the ligands detachfrom the surface of the nanocrystals, which consequentlyprecipitate out. The nanocrystal precipitate can be recovered by
Received: July 26, 2013Revised: September 20, 2013Published: October 7, 2013
deprotonation of the thiol ligands with a base. Dissolution ofinorganic nanocrystals was not observed.The organic ligands that cover the surface of core and core−
shell QDs play multiple roles both during and after syn-thesis.26,42,43 In the synthetic phase the ligands control thereactivity of the precursor and the growth rate, therebydetermining the size and size distribution of the particles.Moreover, the capping ligands passivate the surface atoms andeliminate a substantial amount of defect states, thus preventingaggregation, providing chemical and photophysical stability andcontrolling the solubility of the particles. The electronicstructure of these molecules contributes to the overallelectronic and optical profile of the nanoparticles, passivatingsurface states with consequences on the emission yield.44 Notsurprisingly, the protecting role of the surface ligands isparticularly important in core QDs.45,46
Core−shell QDs are equipped with a layer of anothersemiconductor material covering the surface of the core. Thisinorganic shell is, in its turn, coated with organic ligands, whosepurpose is mainly to provide solubility and prevent aggregation.It is indeed acknowledged17 that the inorganic shell in real QDsis not uniform, and it is likely that in some parts of the coresurface such a layer is very thin or not present at all. It cantherefore be expected that also the properties of core−shellQDs could be influenced by the environment, in spite of theirclaimed stability and insensitivity to the surrounding media.47
Here we report the results of a series of experiments aimed atelucidating the effect of acid on the photophysical properties ofCdSe core and CdSe−ZnS core−shell semiconductor nano-crystals. We also addressed the influence of the thickness of theZnS shell on the changes observed upon addition of acid andstudied the reversibility of such effects by successiveneutralization with a base. We decided to perform theexperiments in organic solvents rather than in water in orderto use the as-prepared QDs, thus avoiding successive exchangereactions with hydrophilic ligands that usually deteriorate thephotophysical properties of the nanocrystals.48 This choice alsoenabled us to exploit in full the flexibility of currently availablesolution-based synthetic methodologies to vary the nature ofthe capping ligands.
■ EXPERIMENTAL SECTIONAll materials and reagents were of the best purity and used withoutfurther purification.CdSe core semiconductor nanocrystal quantum dots were prepared
by reaction of organometallic precursors in high boiling pointnoncoordinating organic solvents in the presence of trioctylphosphineoxide (TOPO) and hexadecylamine (HDA) (cA) following thepublished procedures developed by Peng and co-workers.49
CdSe−ZnS core−shell semiconductor nanocrystal quantum dotsbearing TOPO/HDA (csA1 and csA2) or TOPO/Oleate (csO1 andcsO2) as surface ligands were prepared by overcoating CdSe core witha ZnS shell using the SILAR method.50 CdSe−ZnS nanocrystalscoated with TOPO as surface ligand (csT) were prepared followingthe report by Tomasulo et al. with minor modifications.51 The corediameter and concentration of the quantum dots were estimatedfollowing the published method.52 Shell thickness was estimated asreported in each synthetic protocol.50,51
A Philips CM 100 transmission electron microscope operating at 80kV was used for morphological characterization of quantum dots andfor the aggregation study. For the TEM investigation a Formvar resinfilm supported on conventional copper microgrids was dried undervacuum after deposition of a drop of quantum dots solution in hexane.Spectrophotometric experiments were carried out at room temper-
ature on air-equilibrated solutions of the samples contained in quartz
cuvettes (optical path length of 1 cm). Absorption spectra in the 190−1100 nm range were recorded with a Perkin-Elmer λ45 spectropho-tometer. Fluorescence emission spectra in the 250−900 nm rangewere recorded with a Perkin-Elmer LS50 spectrofluorimeter equippedwith a Hamamatsu R928 photomultiplier. Spectrophotometric andspectrofluorimetric titrations were performed by adding with amicrosyringe small aliquots (typically 5−20 μL) of a concentratedsolution (10−3−10−4 M) of titrating species to a known volume of adilute solution of the titrated species in a quartz cuvette.
■ RESULTS AND DISCUSSIONAll compounds have been fully characterized by means ofabsorption and emission spectroscopy in dilute CHCl3solution; the photophysical properties are gathered in Table1. The size of the nanocrystals was estimated from thewavelength of the maximum of absorption of the excitonband52 and confirmed by TEM images.
We synthesized CdSe quantum dots coated with an organicshell of trioctylphosphine oxide (TOPO) and hexadecylamine(HDA), cA. The size of the nanocrystals, determined from theabsorption spectrum52 and TEM images, was 3.4 nm. Thephotophysical properties are gathered in Table 1.Upon titration of a CHCl3 solution of cA (0.19 μM) with
triflic acid (CF3SO3H) the luminescence of the quantum dotswas completely quenched after addition of 100 equiv of acid(Figure 1), as previously observed for water-soluble CdTenanocrystals.31 No precipitation was observed, even after 3days.30 On the other hand, we observed a slight decrease andhypsochromic shift of the absorption excitonic peak, which iscompletely recovered on addition of base. The differentbehavior with respect to previously investigated systems30 isprobably due to differences in experimental conditions and inparticular on the solvent that we chose for our experiments. Weperformed regular titrations in CHCl3 in very dilute conditions(10−7−10−8 M), and we followed the changes in the emissionspectra until achievement of a plateau of the fluorescence signal.Under these experimental conditions it is possible that thequantum dots reach an equilibrium state, which corresponds toloss of part of the ligands (responsible for the hypsochromicshift in the absorption spectrum)42 and consequent aggregation(without precipitation, responsible for the emission quench-ing).53,54 Successive addition of a base (tributylamine, TBA)should cause deprotonation of the detached ligands andpossibly their reattachment to the surface of the nanocrystals.30
Actually, addition of base causes partial recovery of theluminescence, whose extent depends on the amount of acidpresent in the solution. We performed two experiments: (i) we
added 250 equiv of acid to a solution of CdSe quantum dots,causing complete quenching of the luminescence, and then wetitrated with TBA (Figure 1b). In these experimental conditionsthe luminescence of the nanocrystals was recovered only up to4% of its original value; (ii) we added 75 equiv of acid to asolution of CdSe quantum dots, causing a strong but notcomplete quenching of the luminescence (Figure 1b). Upontitration of this solution with TBA the luminescence wasrestored up to 25% of its original value. These results supportthe hypothesis of an aggregation of the quantum dots afterprotonation of the ligands and their subsequent detachmentfrom the surface of the nanocrystals. After addition of a base,the ligands are deprotonated and able again to bind to thesurface of the quantum dots, thus breaking the aggregates. Theefficiency of this process depends on the extent of aggregationof the nanocrystals, as suggested by our experiments.Coating the inorganic core with a shell of another inorganic
semiconductor material should passivate and stabilize thenanocrystals, rendering them potentially insensitive to theenvironment. Nevertheless, core−shell quantum dots arecovered with a further shell of organic ligands, which areLewis bases and sensitive to acid and base. Therefore, wesynthesized CdSe−ZnS core−shell quantum dots (cs), coatedwith TOPO and HDA (csA1), and we characterized thismaterial in CHCl3 (Table 1). From the exciton absorptionwavelength the dimension of the core could be established;52
the dimension of the shell depends on the syntheticprocedure50 (see Experimental Section for details) and wasconfirmed by TEM images. The emission spectrum ischaracterized by the typical narrow luminescence band,whose maximum depends on the dimensions of the core.In order to investigate the stability of core−shell quantum
dots in the presence of acid, we performed titrationexperiments in CHCl3. Upon addition of CF3SO3H, theabsorption spectrum of the nanocrystals showed only a slight
decrease whereas the luminescence was strongly quenched(Figure 2a): 5000 equivof acid caused a 90% decrease of the
emission quantum yield of csA1. We performed the sametitration with CF3COOH, which is a weaker acid thanCF3SO3H, and we obtained similar results: the only differenceis that more equivalents of acid (20 000) were necessary toquench the 65% of the luminescence of csA1 (see SupportingInformation).Also, in the case of core−shell quantum dots it is likely that
the effect of the acid is to protonate the organic ligands, causingtheir detachment from the surface. The quenching of theluminescence can be due to two reasons. On one hand, as wecould infer also from the spectroscopic properties of ourmaterial, the coverage of the core by the ZnS shell is notperfect,17 and it is likely that part of the CdSe core is protectedfrom the environment only by the layer of organic ligands. As aconsequence, protonation of the ligands can cause exposure ofthe uncoated core of the quantum dots and quenching of theluminescence. Moreover, as for the bare core CdSe, loss ofligands can cause aggregation of the nanocrystals, which is alsoresponsible for quenching of the luminescence.53
Upon addition of TBA the emission was restored up to 35%of the initial value (Figure 2b), and the maximum of theemission band was 3 nm red shifted. Upon leaving the systemat rest for 2 days, the luminescence intensity was enhanced withrespect to the original value (Figure 3). Recovery of theluminescence on addition of a base suggests that upon additionof TBA the protonated free ligands are deprotonated and canbind again on the surface of the quantum dots. At the beginningthe luminescence is only partly restored because of kineticeffects: it is likely that detachment of the protonated ligandscauses aggregation of the quantum dots; after addition of basethe deprotonated ligands can bind again on the surface of thenanocrystals, but this process takes some time because theparticles are aggregated, and their surface is less accessible to
Figure 1. (a) Emission spectra of a solution of cA 1.9 × 10−7 M upontitration with CF3SO3H. (b) (Gray symbols) Emission changes at 580nm upon addition of acid (circles) up to 250 equivalents andsuccessive addition of base (squares); (black symbols) emissionchanges at 580 nm upon addition of acid (circles) up to 75 equivalentsand successive addition of base (squares). Figure 2. Emission spectra of a solution of csA1 6 × 10−8 M upon
addition of CF3SO3H (a) and upon successive addition of TBA (b).(Insets) Emission changes at 607 nm.
the ligands. After 1 day the original emission is restored, andupon leaving the system at rest the emission increases abovethe initial value, possibly for two reasons: (i) the TBA can bindon the surface of the nanocrystals, removing the surface defects;(ii) the mild conditions of the ligand detachment/reattach-ment, with respect to the synthetic experimental conditions,can possibly favor an optimized distribution of the ligands onthe surface of the quantum dots. The first hypothesis can beruled out because addition of TBA to a solution of quantumdots does not cause any spectroscopic changes.Cyclic additions of acid and base to a solution of TOPO/
HDA-coated core−shell quantum dots revealed poor reversi-bility: this observation is consistent with the partialirreversibility of the disgregation process by means of theligands in these experimental conditions.Core−shell quantum dots with different organic ligands on
the surface were also synthesized, namely, CdSe-ZnS coatedwith TOPO (sample csT) and with TOPO and oleate (samplecsO1). These materials have been investigated in CHCl3; themain spectroscopic properties of the two samples are gatheredin Table 1. We performed on these samples the same acid/basetitration experiments carried out on sample csA1, and weobserved that upon addition of CF3SO3H the absorptionspectrum of the nanocrystals is unaffected (csT) or shows onlya slight decrease (csO1), whereas the luminescence is stronglyquenched (Figures 4 and 5): 700 equiv of acid causes a 65%decrease of the emission quantum yield of csT (Figure 4), and35 000 equiv causes a 90% decrease of the emission quantumyield of csO1 (Figure 5).Upon addition of TBA the emission is restored, but the two
samples show a different behavior (Figures 4b and 5b): in thecase of csT, the emission is completely restored, and afterneutralization of the acid in excess it is enhanced with respectto the original value (Figure 4b), whereas sample csO1 showsonly a small recovery of the luminescence (5%) (Figure 5b).The different percentage of recovery in the two samples can betentatively ascribed to the different basicity of the ligands: thestrongest base (TOPO) binds more efficiently to the surface ofthe quantum dot. In the case of sample csT, an increase ofluminescence is observed upon addition of an excess of basewith respect to acid; in fact, this is the only sample that shows a
slight (5%) increase of luminescence upon addition of TBA. Apossible explanation takes into account the shape oftrioctylphosphine oxide and the “volume” that it occupies onthe surface of the QD. It is possible that the coating of thesurface is not optimal, because the monolayer made of TOPOligands may be much more compact in its outer part than in thevicinity of the surface, thereby allowing insertion of the smallmolecule of TBA.From our experiments and from literature data30 we can
interpret the effect of the acid on the quantum dots as aconsequence of the loss of the organic ligands that cover the
Figure 3. Emission spectra of csA1 (solid black line 1) after additionof 5450 equiv of CF3SO3H (dashed black line 2), after addition of 1.5equiv of TBA with respect to CF3SO3H (dotted black line 3), after 12h (solid gray line 4), after 2 days (dashed gray line 5), after 3 days(dotted gray line 6).
Figure 4. (a) Emission spectra of a solution of csT 2.2 × 10−7 M upontitration with CF3SO3H. (b) Emission changes at 592 nm uponaddition of acid (circles) and successive addition of base (squares).
Figure 5. (a) Emission spectra of a solution of csO1 4 × 10−8 M upontitration with CF3SO3H. (b) Emission changes at 638 nm uponaddition of acid (circles) and successive addition of base (squares).
surface of the nanocrystals. As we already pointed out, the effecton the luminescence can be possibly ascribed to two reasons:(i) incomplete coating of the core by the inorganic shell, with aconsequent exposure of the core to the surrounding environ-ment, and/or (ii) aggregation of the quantum dots.53
In order to gain more insight on the effect of the inorganicshell coating, we synthesized three samples of CdSe-ZnSTOPO/HDA (csA2) and TOPO/oleate (csO2) with inorganicshells of different thickness, namely, one (csA2-1 and csO2-1),three (csA2-3 and csO2-3), and five (csA2-5 and csO2-5) ZnSshells. Samples were characterized in CHCl3, and theirspectroscopic properties are gathered in Table 1. Upontitration with acid we observed a quenching of theluminescence; in all cases the amount of acid necessary tocause a comparable quenching in the three samples, however,increases on increasing thickness of the shell (Figure 6). This
observation can be rationalized considering that particles with athicker shell have a larger diameter and are therefore cappedwith a higher number of ligands. On the other hand, theamount of final quenching does not depend on the thickness ofthe inorganic shell, suggesting that this effect is not related tothe exposure of an incompletely coated core but is ratherascribable to aggregation triggered by the acid-induced liganddesorption.All samples of the csO2 series show rather broad emission
peaks (fwhm ≥ 45 nm), indicative of a wide distribution of thedimensions of the nanocrystals. Interestingly, the residualemission peaks at the end of the titration with acid show a redshift of the maximum and a narrower width (Figure 7): thisresult indicates that the quenching effect of the acid is moreefficient on the smaller quantum dots of a distribution.In order to confirm all our hypotheses we performed TEM
experiments on a sample of csO1 (Figure 8a), after addition ofCF3SO3H (Figure 8b), and after successive addition of TBA(Figure 8c). Figure 8b shows that after addition of acid theparticles tend to aggregate: a nonhomogeneous distribution ofthe quantum dots on the grid is clearly visible. After addition ofbase (Figure 8c), the nanoparticles are more homogeneouslydistributed on the surface of the grid, even though still partlyaggregated in small clusters (as expected on the basis of thebehavior observed in solution for this material).
On sample csA2-3 we performed three titrations withCF3SO3H in the same conditions, changing only the amount ofacid added at each point of the titration: we observed that uponincreasing the amount of acid added at each point luminescencequenching is less efficient. In other words, the same totalamount of acid causes a different extent of luminescencequenching, depending on the amount of acid added at eachstep of the titration (Figure 9). Successive addition of TBAshows that also the luminescence recovery depends on how weperformed the titration with the acid, namely, luminescencerecovery is more efficient when the acid has been added in largealiquots, whereas it is almost completely inefficient if the acidhas been added in small aliquots (Figure 9). This behavior canpossibly depend on the kinetics of the aggregation process thatfollows detachment of the ligands: upon addition of acid insmall aliquots, detachment of the ligands causes formation ofsmall aggregates that can in turn aggregate in bigger entities aslong as the titration proceeds. On the contrary, upon additionof a large amount of acid, the partially uncoated nanocrystalsaggregate quickly in smaller aggregates. Therefore, also theeffect of the base is different: the bigger aggregates are moredifficult to disgregate and the luminescence recovery is lessefficient. In this sample we did not observe an increase of theluminescence with respect to the original value on addition of
Figure 6. Emission changes at 590 nm upon addition of CF3SO3H tosamples csA2-1 (white circles), csA2-3 (gray circles), and csA2-5(black circles).
Figure 7. (a) Emission spectra of a solution of csO2-3 3 × 10−8 Mupon titration with CF3SO3H. (b) Normalized emission spectra ofcsO2-3 before (solid line) and after (dashed line) addition of 2400equiv of acid.
Figure 8. TEM images of (a) csO1, (b) csO1 after addition of 1600equiv of CF3SO3H, and (c) after successive addition of 1 equiv of TBAwith respect to tha acid.
TBA, neither after successive addition of native ligands(TOPO, HDA, or both).
■ CONCLUSIONSInorganic semiconductor nanocrystals are attracting more andmore interest for their peculiar spectroscopic properties andalso for their stability and insensitivity to the surroundingenvironment with respect to organic dyes, particularly as far ascore−shell systems are concerned. In recent years it isbecoming more evident that the surface properties of thesematerials are very important, because it has been observed thatoptical properties of quantum dots can depend on their localenvironment.55,56 In this study we explored the behavior ofcore−shell quantum dots in acid environment, comparing thesame materials originating from different synthesis, materialswith different organic ligands, and materials with different shellthicknesses. From our results it is evident that the behavior ofthe systems is influenced by several parameters, but we canreasonably conclude that core−shell quantum dots are sensitiveto the acidity of the environment, because of the presence ofthe organic ligands on their surface. This behavior must betaken into account in designing quantum dots-based systemsthat are supposed to work in the presence of acid or when usingquantum dots as scaffolds for pH sensors.
■ ASSOCIATED CONTENT*S Supporting InformationAbsorption and emission spectra, TEM, and titration experi-ments. This material is available free of charge via the Internetat http://pubs.acs.org.
■ AUTHOR INFORMATIONCorresponding Author*Phone: +39 051 2099463. Fax: +39 051 2099456. E-mail:[email protected] ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
Financial support from the EU (FP7-NMP project Hysens, No.263091), MIUR (PRIN 2010CX2TLM), and the University ofBologna is gratefully acknowledged.
■ REFERENCES(1) Rogach, A. L. Semiconductor Nanocrystal Quantum Dots; Springer:Wien, Austria, 2008.(2) Brus, L. Electronic wave functions in semiconductor clusters:experiment and theory. J. Phys. Chem. 1986, 90, 2555−2560.(3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis andcharacterization of monodisperse nanocrystals and close-packednanocrystal assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545−610.(4) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, andQuantum Dots. Science 1996, 271, 933−937.(5) Efros, A. L.; Rosen, M. The electronic structure of semiconductornanocrystals. Annu. Rev. Mater. Sci. 2000, 30, 475−521.(6) Artemyev, M. V.; Woggon, U.; Wannemacher, R.; Jaschinski, H.;Langbein, W. Light Trapped in a Photonic Dot: Microspheres Act as aCavity for Quantum Dot Emission. Nano Lett. 2001, 1, 309−314.(7) Smith, A. M.; Nie, S. Semiconductor Nanocrystals: Structure,Properties, and Band Gap Engineering. Acc. Chem. Res. 2010, 43, 190−200.(8) Somers, R. C.; Bawendi, M. G.; Nocera, D. G. CdSe nanocrystalbased chem-/bio- sensors. Chem. Soc. Rev. 2007, 36, 579−591.(9) Gill, R.; Zayats, M.; Willner, I. Semiconductor Quantum Dots forBioanalysis. Angew. Chem., Int. Ed. 2008, 47, 7602−7625.(10) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Quantum-dot-taggedmicrobeads for multiplexed optical coding of biomolecules. Nat.Biotechnol. 2001, 19, 631−635.(11) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V.Prospects of Colloidal Nanocrystals for Electronic and OptoelectronicApplications. Chem. Rev. 2010, 110, 389−458.(12) Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F.Full Color Emission from II−VI Semiconductor Quantum Dot−Polymer Composites. Adv. Mater. 2000, 12, 1102−1105.(13) Demir, H. V.; Nizamoglu, S.; Erdem, T.; Mutlugun, E.; Gaponik,N.; Eychmuller, A. Quantum dot integrated LEDs using photonic andexcitonic color conversion. Nano Today 2011, 6, 632−647.(14) Kamat, P. V. Quantum Dot Solar Cells. SemiconductorNanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112,18737−18753.(15) Ruhle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized SolarCells. ChemPhysChem 2010, 11, 2290−2304.(16) Resch-Genger, U.; Grabolle, M.; Cavaliere-Jaricot, S.; Nitschke,R.; Nann, T. Quantum dots versus organic dyes as fluorescent labels.Nat. Methods 2008, 5, 763−775.(17) McBride, J.; Treadway, J.; Feldman, L. C.; Pennycook, S. J.;Rosenthal, S. J. Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures. Nano Lett. 2006, 6, 1496−1501.(18) Wang, X.; Qu, L.; Zhang, J.; Peng, X.; Xiao, M. Surface-RelatedEmission in Highly Luminescent CdSe Quantum Dots. Nano Lett.2003, 3, 1103−1106.(19) Hines, M. A.; Guyot-Sionnest, P. Synthesis and Characterizationof Strongly Luminescing ZnS-Capped CdSe Nanocrystals. J. Phys.Chem. 1996, 100, 468−471.(20) Amelia, M.; Lincheneau, C.; Silvi, S.; Credi, A. Electrochemicalproperties of CdSe and CdTe quantum dots. Chem. Soc. Rev. 2012, 17,5728−5743.(21) Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.;Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. “Giant” MultishellCdSe Nanocrystal Quantum Dots with Suppressed Blinking. J. Am.Chem. Soc. 2008, 130, 5026−2027.(22) Galland, C.; Ghosh, Y.; Steinbruck, A.; Sykora, M.;Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Two types ofluminescence blinking revealed by spectroelectrochemistry of singlequantum dots. Nature 2011, 479, 203−207.
Figure 9. Emission changes upon addition of triflic acid (circles) andTBA (squares) in different aliquots to sample csA2-3: white, gray, andblack symbols are referred to additions of increasing amount of acidper aliquot.
(23) Gooding, A. K.; Gomez, D. E.; Mulvaney, P. The Effects ofElectron and Hole Injection on the Photoluminescence of CdSe/CdS/ZnS Nanocrystal Monolayers. ACS Nano 2008, 2, 669−676.(24) Tomasulo, M.; Yildiz, I.; Raymo, F. M. pH-Sensitive QuantumDots. J. Phys. Chem. B 2006, 110, 3853−3855.(25) Snee, P. T.; Somers, R. C.; Nair, G.; Zimmer, J. P.; Bawendi, M.G.; Nocera, D. G. A Ratiometric CdSe/ZnS Nanocrystal pH Sensor. J.Am. Chem. Soc. 2006, 128, 13320−13321.(26) Susha, A. S.; Munoz Javier, A.; Parak, W. J.; Rogach, A. L.Luminescent CdTe nanocrystals as ion probes and pH sensors inaqueous solutions. Colloids Surf., A 2006, 281, 40−43.(27) Smith, A. M.; Duan, H.; Rhyner, M. N.; Ruan, G.; Nie, S. Asystematic examination of surface coatings on the optical and chemicalproperties of semiconductor quantum dots. Phys. Chem. Chem. Phys.2006, 8, 3895−3903.(28) Liu, Y.-S.; Sun, Y.; Vernier, P. T.; Liang, C.-H.; Chong, S. Y. C.;Gundersen, M. A. pH-Sensitive Photoluminescence of CdSe/ZnSe/ZnS Quantum Dots in Human Ovarian Cancer Cells. J. Phys. Chem. C2007, 111, 2872−2878.(29) Mohs, A. M.; Duan, H.; Kairdolf, B. A.; Smith, A. M.; Nie, S.Proton-Resistant Quantum Dots: Stability in Gastrointestinal Fluidsand Implications for Oral Delivery of Nanoparticle Agents. Nano Res.2009, 2, 500−508.(30) Aldana, J.; Lavelle, N.; Wang, Y.; Peng, X. Size-DependentDissociation pH of Thiolate Ligands from Cadmium ChalcogenideNanocrystals. J. Am. Chem. Soc. 2005, 127, 2496−2504.(31) Zhang, Y.; Mi, L.; Wang, P.-N.; Mab, J.; Chen, J.-Y. pH-dependent aggregation and photoluminescence behavior of thiol-capped CdTe quantum dots in aqueous solutions. J. Lumin. 2008, 128,1948−1951.(32) Gao, X.; Chan, W. C. W.; Nie, S. Quantum-dot nanocrystals forultrasensitive biological labeling and multicolor optical encoding. J.Biomed. Opt. 2002, 7, 532−537.(33) Ruedas-Rama, M. J.; Orte, A.; Hall, E. A. H.; Alvarez-Pez, J. M.;Talavera, E. M. Quantum dot photoluminescence lifetime-based pHnanosensor. Chem. Commun. 2011, 47, 2898−2900.(34) Maule, C.; Goncalves, H.; Mendonca, C.; Sampaio, P.; Estevesda Silva, J. C. G.; Jorge, P. Wavelength encoded analytical imaging andfiber optic sensing with pH sensitive CdTe quantum dots. Talanta2010, 80, 1932−1938.(35) Yu, D.; Wang, Z.; Liu, Y.; Jin, L.; Cheng, Y.; Zhou, J.; Cao, S.Quantum dot-based pH probe for quick study of enzyme reactionkinetics. Enzyme Microb. Technol. 2007, 41, 127−132.(36) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H.Quantum dot bioconjugates for imaging, labelling and sensing. Nat.Mater. 2005, 4, 435−446.(37) Mei, B. C.; Susumu, K.; Medintz, I. L.; Delehanty, J. B.;Mountziaris, T. J.; Mattoussi, H. Modular poly(ethylene glycol)ligands for biocompatible semiconductor and gold nanocrystals withextended pH and ionic stability. J. Mater. Chem. 2008, 18, 4949−4958.(38) Gao, M.; Kirstein, S.; Mohwald, H.; Rogach, A. L.; Kornowski,A.; Eychmuller, A.; Weller, H. Strongly Photoluminescent CdTeNanocrystals by Proper Surface Modification. J. Phys. Chem. B 1998,102, 8360−8363.(39) Mandal, A.; Tamai, N. Influence of Acid on LuminescenceProperties of Thioglycolic Acid-Capped CdTe Quantum Dots. J. Phys.Chem. C 2008, 112, 8244−8250.(40) Deng, Z.; Zhang, Y.; Yue, J.; Tang, F.; Wei, Q. Green andOrange CdTe Quantum Dots as Effective pH-Sensitive FluorescentProbes for Dual Simultaneous and Independent Detection of Viruses.J. Phys. Chem. B 2007, 111, 12024−12031.(41) Zhang, H.; Zhou, Z.; Yang, B.; Gao, M. The Influence ofCarboxyl Groups on the Photoluminescence of MercaptocarboxylicAcid-Stabilized CdTe Nanoparticles. J. Phys. Chem. B 2003, 107, 8−13.(42) Bullen, C.; Mulvaney, P. The Effects of Chemisorption on theLuminescence of CdSe Quantum Dots. Langmuir 2006, 22, 3007−3013.
(43) Kalyuzhny, G.; Murray, R. W. Ligand Effects on OpticalProperties of CdSe Nanocrystals. J. Phys. Chem. B 2005, 109, 7012−7021.(44) Green, M. The nature of quantum dot capping ligands. J. Mater.Chem. 2010, 20, 5797−5809.(45) Guyot-Sionnest, P.; Weherenberg, B.; Yu, D. Intrabandrelaxation in CdSe nanocrystals and the strong influence of thesurface ligands. J. Chem. Phys. 2005, 123, 074709−1−074709−7.(46) Kilina, S.; Velizhanin, K. A.; Ivanov, S.; Prezhdo, O. V.; Tretiak,S. Surface Ligands Increase Photoexcitation Relaxation Rates in CdSeQuantum Dots. ACS Nano 2012, 6, 6515−6524.(47) Grabolle, M.; Ziegler, J.; Merkulov, A.; Nann, T.; Resch-Genger,U. Stability and fluorescence quantum yield of CdSe-ZnS quantumdots - Influence of the thickness of the ZnS shell. Ann. N.Y. Acad. Sci.2008, 1130, 235−241.(48) Uyeda, H. T.; Medintz, I. L.; Jaiswal, J. K.; Simon, S. M.;Mattoussi, H. Synthesis of Compact Multidentate Ligands to PrepareStable Hydrophilic Quantum Dot Fluorophores. J. Am. Chem. Soc.2005, 127, 3870−3878.(49) Peng, Z. A.; Peng, X. Formation of High-Quality CdTe, CdSe,and CdS Nanocrystals Using CdO as Precursor. J. Am. Chem. Soc.2001, 123, 183−184.(50) Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.;Johnson, M. B.; Peng, X. Large-Scale Synthesis of NearlyMonodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-StableReagents via Successive Ion Layer Adsorption and Reaction. J. Am.Chem. Soc. 2003, 125, 12567−12575.(51) Tomasulo, M.; Yildiz, I.; Kaanumalle, S. L.; Raymo, F. M. pH-Sensitive Ligand for Luminescent Quantum Dots. Langmuir 2006, 22,10284−10290.(52) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. ExperimentalDetermination of the Extinction Coefficient of CdTe, CdSe, andCdS Nanocrystals. Chem. Mater. 2003, 15, 2854−2860.(53) Komoto, A.; Maenosono, S.; Yamaguchi, Y. OscillatingFluorescence in an Unstable Colloidal Dispersion of CdSe/ZnSCore/Shell Quantum Dots. Langmuir 2004, 20, 8916−8923.(54) Proton transfer to the QDs in the excited state could possiblytake place and luminescence in the protonated nanocrystal should bequenched by charge trapping in the easy to reduce protonated surfacesites. Excited-state proton transfer, however, should occur in collisionalencounters between the excited QDs and the proton donors andwould be governed by Stern−Volmer-type kinetics. In ourexperimental conditions the amount of collisional quenching wouldbe <1% even assuming that the process is diffusion controlled.(55) Blum, A. S.L; Moore, M. H.; Ratna, B. R. Quantum DotFluorescence as a Function of Alkyl Chain Length in AqueousEnvironments. Langmuir 2008, 24, 9194−9197.(56) Pechstedt, K.; Whittle, T.; Baumberg, J.; Melvin, T. Photo-luminescence of Colloidal CdSe/ZnS Quantum Dots: The CriticalEffect of Water Molecules. J. Phys. Chem. C 2010, 114, 12069−12077.
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