Student Project Abstracts 2005 - Pluto - University of Washington

H 2 H 2 H 2 HC2 HC2CHC2 HC2 HHC22N CCCCCCH C2 HC2C CH2 H2 H C2CHH2 H2 H 32H 2 H 2 H 2 HC2 HC2CHC2 HC2 HHC22N CCCCCCH C2 HC2C CH2 H2 HC2CHH2 H2 H 32The QY was enhanced more by the addition of ODA than BA.To investigate the reversibility of ligand binding, ODA wasadded to CdSe NCs already partially exchanged with ODT. The QYof CdSe NCs in 1.2E-5 M ODT was ~6%. Figure 6 shows that uponaddition of ~0.1M ODA, the QY was increased to ~21%. The QYof CdSe NCs in 6.2E-6 M ODT was ~10% and figure 7 shows thatthe addition of ~0.04 M ODA increased QY up to ~35%.Figure 8, 9, and 10 show the H-NMR 9,10 spectra that wasperformed to attempt to determine the relative amounts of eachFigure 10. shows the structure of 1-octadecanethiol (a) andcorresponding peak assignments (b) on the solution state H-NMRspectra of ODT. Figure 10 (c) shows the H-NMR spectra whenODT is bound to 1E-5 mmol CdSe in deuterated chloroform.In the case of ODA binding to CdSe, the triplet peak (b) at~2.66 ppm corresponding to the alpha-protons (circled in Figure9) is broadened. The pentet at ~3.19 ppm leans towards the upfieldpeaks indicating that it is interacting with the ODA. *Acetone andstearicFigure 10acid impurities. o Peaks are a result of amine bound to NC.Figure 10 shows the structure of 1-octadecanethiol (a) andcorresponding peak assignments (b) on the solution state H-NMRspectra of ODT. Figure 10 (c) shows the H-NMR spectra whenODT Figure is 10 bound to 1E-5 mmol CdSe in deuterated chloroform.2 4 51 3 62 4 54In 1 H-NMR studies of ODT, the proton on the sulfur atom(circled region in Figure 10) is presumed to have a chemical shiftnear the methylene peak at ~1.2 ppm making the peak difficult toresolve.DISCUSSIONWe hypothesized that the ligand binding could be describedby a Langmuir isotherm (eqn 1). The Langmuir isotherm relatesthe fractional surface coverage, θ, of ligand on surface sites to theconcentration of ligand in solution and can be derived from anequilibrium reaction given the equilibrium binding constant, K,and ligand concentration, [ligand].ligand Figure in 8, solution 9, and 10 that show is free the or H-NMR bound to 9,10 a spectra CdSe NC. that H-NMR was performed to attempt to determine thespectra relative of amounts 1E-5 mmol of each washed ligand CdSe in solution NCs, ODA, that is ODA free or bound bound to to a CdSe NC. H-NMR spectra of1E-5 mmol washed CdSe NCs, ODA, ODA bound to CdSe, ODT, and ODT bound to CdSe inCdSe,deuteratedODT,chloroformand ODT boundwere conducted.to CdSe in deuterated chloroformwere conducted.Figure 8 8, 9, and 10 show the H-NMR 9,10 spectra that was performed to attempt to determine therelative amounts of each ligand in solution that is free or bound to a CdSe NC. H-NMR spectra of1E-5 mmol washed CdSe NCs, ODA, ODA bound to CdSe, ODT, Figure and 8 ODT shows bound the 300 to CdSe MHzindeuterated chloroform were conducted.solution state H-NMR spectra of 1E-5mmol CdSe NCs in deuteratedFigure 8In order to use the isotherm, four assumptions were made: (1)the reaction is reversible, (2) only one monolayer can be formed,Figure 9Figure 8 shows the 300 MHz(a) Figure 8. shows the 300 MHz solution state H-NMR spectra solution (3) binding state H-NMR of a ligand spectra to of a 1E-5 surface site does not affect the bindingof 1E-5 mmol CdSe NCs in deuterated chloroform.bdmmol at any CdSe other NCs sites, in and deuterated (4) the QY is linearly dependent on theeacfractional surface coverage, θ. We also assumed that the NCsfFigure 9started with all empty surface sites, which is not true because(a)there are stearic acid, TOPO, ODA bound to the surface.bdFigure 9. shows the structure of 1-octadecylamine (a) ande A more accurate model would use a multiple componenta corresponding c peak assignments b (b) on the colution d,e state H-NMR Langmuir isotherm in which the ligand already bound to the surfaceis taken (a) into and account. corresponding The fluorescence quenching that occursspectra of ODA. Figure 9 (c) shows the H-NMR spectrafFigure 9 showswhenfstructure of 1-(b)aoctadecylamineODA is bound to IE-5 mmol CdSe deuterated chloroform.cpeak assignments (b) on the solutionat high amine concentrations does not agree with the assumptionstate H-NMR spectra of ODA.Figure (4) that 9 (c) the shows QY increases the H-NMR linearly as the surface is passivated withbd,e* ofspectra Figure more when 9 amine. shows ODA the For is structure bound the amine to of 1E-5 1- curves the fits were made with data at(b)o ammol octadecylamine CdSe deuterated (a) and correspondingchloroform.(c)camine concentrations below the solution saturation point of ODApeak assignments (b) on the solutionstate (~0.01 H-NMR M). spectra of ODA.In the case of ODA binding to CdSe, the triplet peak (b) at ~2.66 Figure ppm corresponding 9 To (c) improve shows to the themodel, H-NMR alphaprotons(circled in Figure 9) is broadened. The pentet at ~3.19 ppm leans towards the upfield peaksfits with the Hill equation (eqn 2)indicating that it is interacting with* ospectra when ODA is bound to 1E-5the ODA. o *Acetone and stearic mmol were acid CdSe impurities. performed. in deuterated The o chloroform. Peaks Hill are equation a is widely used as an empiricalresult (c) of amine bound to NC.fitting function when some degree of deviation from the Langmuirisotherm assumptions is expected.In the case of ODA binding to CdSe, the triplet peak (b) at ~2.66 ppm corresponding to the alphaprotons(circled in Figure 9) is broadened. The pentet at ~3.19 ppm leans towards the upfield peaksindicating that it is interacting with the ODA. *Acetone and stearic acid impurities. o Peaks are aresult of amine bound to NC.NGTable 1 Langmuir isotherm- K Hill equation- K Hill equation- nCdSe + ODT 106980 +/- 24500 94877 2CdSe + BT 101800+/- 4300 186470 2CdSe + ODA 2001.7 +/- 522 4180 1CdSe + BA 1787.3 +/- 769 2000 1Table 1. shows the equilibrium binding constant (K) valuesfound for least-squares fits of the Langmuir isothermand Hill equations to the experimental data.1 3 CMDITR 6 Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 91