The Journal of
Physical Chemistry
0 Copyright. 1992, by the American Chemical Societj
VOLUME 96, NUMBER 26 DECEMBER 24,1992
LETTERS Deactivation of Q-CdS Photoluminescence through Polynucleotide Surface Binding Sbelli R. Bigham and Jeffery L. Coffer* Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129 (Received: July 8, 1992; In Final Form: November 9, 1992) The intrinsic interaction between cadmium hydroxide-layered Q-CdS clusters and polynucleotides has been examined via photoluminescence (PL) spectroscopy. For semiconductor clusters of 40-A average diameter and a narrow luminescence maximum near 480 nm, quenching of this emission can be accomplished by the addition of polynucleotides such as deoxyribonucleic acid (DNA) from E. coli or polyadenylic acid (poly[A]). The observed PL area changes can be fit to a Perrin model, with the calculated volume of the quenching sphere for poly[A] being more than an order of magnitude for that of DNA, 4.5 X 10' A3 versus 4.4 X lo3 A3. After exposure to polynucleotides, these CdS surfaces are shown to be reactive, as attempts to grow ZnS layers at the interface result in an overall enhancement of trap emission in the 500-700-nm region.
Introduction Control of particle size and surface structure continues to be of interest in the investigation of stable quantum confined ("Qsized") semiconductor clusters.' The surface of these particles is important to semiconductor cluster photophysics, since (1) the charge carriers can localize on surface defects and (2) a large percentage of the total atom composition exists on the surface of these clusters. Several different approaches to the deliberate surface modification of these Qstate materials have km reported. These include a layering of cadmium hydroxide on CdS2and ZnS on CdSe3to remove the defect sites responsible for lower energy trap emission. The binding of amines,' (dimethylamino). methylferrocene? and lanthanide &diketonate complexes6 to the surface of Q-CdS enhances its overall photoluminescence (PL), while aliphatic thiols' and ammonia8reduce the number of surface defects when these mgenta are present during particle formation. One recent approach under investigation in our laboratories is the usc of polynuclsotide stabilizers as a uscfd and unique probe of the Q-size semiconductor cluster/stabilizer interface. Previously, we have reported the synthesis and characterization of To whom correspondence should be addressed.
0022-3654/92/2096-10581$03.00/0
50-A-diameter Q-CdS semiconductor clusters in the presence of calf thymus DNA, polyadenylic acid (poly[A]), a polycytidylic acid (poly[C]) and noted that the nature of the semiconductor luminescence is sensitive to the type of polynucleotide utilized? For the case of Q C d S synthesized in the presence of calf thymus DNA, existing experimental results suggest a physical picture of a strong cluster-DNA interaction.1° This, in turn, brings about the question of the intrinsic affinity of polynucleotides for semiconductor surfaces. Thus, we have subsequently studied the interaction between hydroxide-layered Q-CdS clusters and the polynucleotides DNA and polyadenylic acid. The parent Q-CdS clusters in this case exhibit narrow band edge PL in the range 465-500 nm, We demonstrate that the addition of polynucleotides, E. coli DNA and poly[A], quenches overall emission in this region through a modification of the cadmium hydroxide-layered C I S surface. Subsequent attempts to form ZnS at the modified cluster surface show that the surface is still chemically accessible, giving rise to enhancement of defect PL over a broad range (500-700 nm) rather than the elimination of surface traps. Experimental Section Q-CdS Colloids. A 50-mL solution of 2 X lo4 M Cd(C104)2-6H20 (10 pL of a 1 M aqueous solution; Johnson-Matthey, 0 1992 American Chemical Society
10582 The Journal of Physical Chemistry, Vol. 96, No. 26, I992
electronic grade) and 2 X lo4 M (NaPO& was prepared according to the method of Henglein and co-workers2with a starting pH of 9.8. The pH adjustments were made by microliter addition of 1 M NaOH (15-25 pL, EM Science). Solutions were purged with N2 before sulfide addition. A stoichiometric amount of sulfide was introduced as H2S (99.5+%, Aldrich) in a 1:l sulfide to cadmium ratio, and solutions were left to form CdS for at least 15 min before spectra were recorded and activation of samples. Colloids were activated by addition of 1 M NaOH (30-50 pL) followed by addition of Cd2+(40-55 pL) until optimal PL intensity was reached. A 400-pL aliquot was removed and diluted to 2.6 mL, with a total volume of 3 mL typically achieved after polynucleotide addition. Poly~~leotides Approximately 5 mg of E. Coli DNA (sodium salt, Sigma) or poly[A] (potassium salt, Sigma) was dissolved in 5 mL of distilled deionized H 2 0 (obtained from Millipore cartridge filter system or Mallinckrodt HPLC grade). Polynucleotides such as poly[A] and E. Coli DNA are slow to dissolve and were allowed to stand at least 30 min, whereupon through mixing aided in obtaining homogeneous solutions. Relative molar concentrations were obtained spectrophotometrically by employing an c value of 6600 and 15400 M-' cm-l for DNA and poly[A], respectively, at X260.11 Polynucleotides were added to colloids via a microliter syringe, with PL spectra recorded after each addition. Measurements of integrated luminescence intensity as a function of quencher concentration were carried out in the region 440-700 nm. Alternative use of the PL maximum at 480 nm yielded similar, satisfactory results (see Results and Discussion). Apparatus. Absorption measurements were made using a HP 8452A diode array spectrophotometer with quartz 1-cm-path length cuvettes. PL spectra were recorded using a Spex Fluorolog-2.0 0.22-m double spectrometer with a constant excitation wavelength of 375 nm. Emission spectra were corrected for fluctuations in photomultiplier tube response. A Coming 140 pH meter with a Markson combination electrode was used for pH measurements. Zns Growth. Aqueous solutions (0.006M) Of Zn(C104)24H20 (98.9%, Johnson-Matthey) and Na2S*9H20(98% Aldrich) were prepared for addition to the colloid/nucleotide mixtures cited above. A 25-pL sample of the Znz+solution, immediately followed by 25 p L of the S2- solution, was added in an alternate fashion to cluster/nucleotide solutions while being purged under N2; PL spectra were recorded immediately after Sz- addition. Aliquots of Zn2+and S2- were added until a ratio of 6:l ZnS:CdS (based on initial sulfide concentration) was reached.
Results and Discussion Previous studies by Henglein and -workers have demonstrated that optimal addition of NaOH and excess Cd2+to hexametaphosphate (HMP)-stabilized Q-CdS semiconductor clusters results in a cadmium hydroxide-layered material that is essentially defect-free, with only a narrow luminescence band observed for these clwsrers (465-500 nm).2 This emission is rather loosely described as "band edge" luminescence (BEL), as a consequenceof a slight Stokes shift from the absorption onsets2 The emission peak maximum is shifted depending on the starting pH before activation of the cluster, and it has been established previously that for this particular type of QCds the position of the fluorwnce maximum after activation is a sensitive probe of average particle size.1b,2 Spectroscopic data on such Q-CdS clusters prepared in our laboratories by this method revealed an absorption edge of 480 nm, and a photoluminescence (PL) maximum near 475 nm, consistent with an average particle diameter of 40 A. With incremental addition of E. Coli DNA to a solution of these Q-CdS clusters, this narrow band edge PL was quenched dramatically, as shown in Figure 1. After addition of 400 pL of 4.4 X M E. Coli DNA (molar nucleotide basis), the narrow BEL peak had disappeared and only weak, broad trap emission with a maximum near 600 nm was observed. Interestingly, these quenched spectra resemble that of Q-CdS prepared in the presence of deoxyribonucleic acids with an overall lower quantum yield. This is viewed as a strong confirmation of the role of the DNA in influencing
Letters I
-.---. I
-----
400
510
0.0 DNA 8.24e-05 M 2.38e-04 M 5.8%-04 M
620
730
Wavelength (nm) Figure 1. Effect of increasing DNA addition on the photoluminescence spectrum of cadmium hydroxide-layered Q-CdS semiconductor clusters. DNA concentrations are expressed on a molar nucleotide basis.
the surface states of the semiconductor. Controls done with the addition of 400 pL of distilled, deionized HzO, 4.4 X M M Tris buffer show that this effect is not NaOH, and 1 X due simply to dilution or quenching as a result of the presence of counterions or base. Relevant optical absorption spectra reveal a slight increase in absorption below 300 nm due to polynucleotide addition; however, the absorption onset does not change, indicating that the cluster is not corroding with the addition of polynucleotides. Examining the emission properties of these samples again after aging for (a) 1 day and (b) 1 week revealed no indication of reformation of the activated (strongly luminescent) phase, providing evidence for an intrinsic affhity of polynucleotides for Q-CdS cluster surfaces and its ability to modify the surface of Q-CdS. Addition of 8.9 X 1W M poly[A] to these QCds colloids shows similar PL quenching behavior as with E. Coli DNA (Figure 1). However, with poly[A] comparable quenching of this narrow band edge PL is accomplished at far lower polynucleotide concentrations (factor of 9 less). The fully quenched spectrum in this case possesses a broad maximum near 500 nm, closer to the 540-nm maximum observed in the emission spectrum of Q-CdS in the presence of poly[A] .9 As before, the absorption edge does not change with poly[A] addition. Attempts to fit changes in nucleotideinduced Q-CdS emission intensity to a simple Stem-Voher relationship (Zo/l)demonstrate curvature at high quencher concentrations. Such curvature has been previously attributed to multiple binding sites for fluorescence quenching in a polynucleotide matrix by Barton and co-workers.I2 Use of a Perrin f~rmalism,'~ on the other hand, provides a better fit of the data. The Perrin model is valid for energy transfer between donor-acceptor components unable to change spatial positions with regard to one another on the time scale of the quenching process. The assumption is made here that an effective quenching sphere exists from a cluster surface. The Perrin relationship is given in the following equation
where Io and Z are integrated emission intensities in the absence and presence of polynucleotide, respectively, Vis the volume of the quenching sphere in cubic centimeters, No is Avagadro's number, and [Q] is molar concentration of quencher (polynucleotide). A plot of In versus [Q] should demonstrate linear behavior with a slope equal to NoV. Figure 2 illustrates the linear behavior of In (Io/Z)for Q-CdS emission after addition of (i) E. Coli DNA and (ii) poly[A], where the integrated luminescence intensities I are measured in the region 440-700 nm. As shown in this plot, the poly[A]-quenched sample exhibits a much steeper slope, corresponding to a larger volume of the effective quenching sphere for cluster-poly[A] electron transfer than for cluster-DNA electron transfer. Analysis of this data in light of eq 2 allows for a calculation of the volume of this sphere. The average effective quenching volume is thus 4.5 x lo4 A3 for Q-CdS/poly[A] and 4.4 X lo3A' for the Q-CdS interaction with DNA. If one determines I and Iovalues by monitoring the Q-CdS
Letters
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10583
0
0.0002
0.0004
0.0006
Polynucleotide Concentration ( M ) F I2. Pemn relationship for the quenching of the photoluminescence spectrum of cadmium hydroxide-layeredQ-CdS semiconductor clusters with the polynucleotides DNA and poly[A]. Polynucleotide concentrations cited are with respect to a molar nucleotide basis.
emission maximum at the single wavelength of 480 nm instead of integrated areas, rather similar observations are found, e.g., 4.3 X lo4 A3 for Q-CdS/poly[A] and 3.9 X lo3 AS for QCdS/DNA. It follows that the larger quenching sphere for the case of single-stranded poly[A] relative to double-stranded E. Coli DNA is a consequence of more accessible purine residues in the reaction medium, resulting in a more facile electron-transfer process for the case of the interaction between polyadenylic acid and Q-CdS.I4 In terms of chemical interactions, there are two likely types of sites responsible for this process. The first is a perturbation of the Cd-OH surface sites via hydrogen-bonding interactions with the anionic phosphate groups of the polynucleotide. Second, the recent studies of Weller and co-workers suggest that perturbation of passivated anionic sulfide centers at the QCdS surface (the sites presumably responsible for hole traps) by hydrogen-bonding solvents depassivates the surface and thus makes detection of "excitonic" luminescence impossible.'s Thus, hydrogen-bonding interactions between the N-H moiety of accessible bases of adenine, guanine, and cytosine and these anionic sulfide sites are a likely candidate for diminution of the observed fluorescence. Furthermore, the rather radical difference between single-stranded poly[A] and double helical DNA can be viewed not only in terms of the greater accessibility of the N-H bonds of the poly[A] to the semiconductor surface but also as a consequence of OH groups at the ribose sugars of the RNA participating in analogous hydrogen-bonding interactions. In order to evaluate the chemical reactivity of these modified Q-CdS semiconductor cluster surfaces, experiments were attempted involving the growth of ZnS layers. Previous studies by Brus and co-workers on the growth of ZnS on CdSe in solution revealed that ZnS layers found under such conditions are epitaxial, as broad trap emission is eliminated and replaced by narrow band edge type emission at 530 nm (near the absorption onset) with a high quantum yield.3 For the polynucleotide-modified Q-CdS samples studied here, the addition of aliquots of ZnZ+followed by Sz-does result in a dramatic enhancement in emission intensity in the 50W00-nm region (Figure 3); however, such observations are not due to an elimination of surface traps but rather an accentuation of defect emission in this region. This emission is also not due to the intrinsic emission of ZnS possibly stabilized by DNA, as control experiments in this case on ZnS clusters deliberately synthesized in the presence of DNA16 exhibit no detectable fluorescence when excited at 375 nm (sub-bandgap excitation). This enhancement is metastable, as reflected in a loss of PL intensity after ca. 24 h and complete disappearance after 1 month. With regard to observed changes in optical absorption, there is increased optical density at lower wavelengths (300325-nm region), similar to previous observations for mixed Cd,Zn,S crystallites in dhexadecyl phosphate vesicles (DHP).I7 This observed enhanced luminescence here is somewhat similar to that previously reported for solid solutions of Cd,Zn$.17J8 Since there exists a large number of defect sites at the modified CdS surface (vide infra), the addition of ZnZ+/S2-in this case likely forms a mixed Cd,Zn,S phase at the interface whose exact composition is ill-defined; this is particularly true given the
400
510
620
-30
Wavelength (nm) Figure 3. PL spectra for a modified cadmium hydroxide-layeredQ-CdS cluster surface demonstrating the effect of alternating addition of 0.006 M aqueous solutions of Zn2+/S2-: (i) 0, (ii) 25, (iii) 50, and (iv) 75 pL of each reagent.
presence of Cd-O groups and other possible types of surface defects. One possibility is that "islands" of ZnS on CdS are formed, similar to that proposed for Cd,Zn$ particles in DHP vesicles." We also cannot, however, absolutely rule out some slight contribution to the enhanced luminescence yield from Zn*+/ S2--induced dissolution processes whereby small free mixed particles of cadmium/zinc sulfide can be formed. As a consequence of these observations, we have developed a physical model for the Q-CdS/polynucleotide/ZnS interaction. Given the presence of some lower energy defect emission (A > 500 nm) in the original Q-CdS material (Figure l), it is believed that the cluster particle is not completely passivated with hydroxide moieties. As outlined above, addition of a polynucleic acid to this surface gives rise to two types of hydrogen-bonding interactions: (a) between nucleotide phosphate and the OH bound to cadmium; (b) between nucleobase N-H (as well as ribose 0-H in the case of RNA) and anionic sulfide hole traps at the Q-CdS surface. All of these modified surface defects should yield basic, perhaps negatively charged sites which would serve as reactive sites for subsequent zinc ion addition. In summary,luminescence in hydroxide-layered Q-CdS cluster surfaces can be quenched by the addition of polynucleotides, resulting in an interface which is accessible to subsequent reactions with zinc and cadmium ions. Interestingly, the observed nucleotide-induced quenching processes can be fit successfully to a Perrin-type model. Acknowledgment. The authors gratefully acknowledge financial support from the Robert A. Welch Foundation. We also thank a reviewer for bringing ref 15 to our attention and for helpful comments.
References and Notes (1) (a) Brus, L. E. J . Phys. Chem. 1986,90,2555. (b) Henglein, A. Chem. Reu. 1989, 89, 1861. (c) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990,23, 183. (d) Stucky, G. D.; Mac Dougall, J. E. Science 1990,247,669. (e) Wang, Y.; Herron, N. J . Phys. Chem. 1991, 95, 525. (f) Steigerwald, M. L.; Brus, L. E. Annu. Rev. Mater. Sci. 1989, 19, 471. (2) Spanhel, L.; Haase, M.; Weller, J.; Henglein, A. J . Am. Chem. Soc. 1987, 109, 5649. (3) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J . Am. Chem. Soc. 1990, 112, 1327. (4) Dannhauser, T.; ONeil, M.; Johansson, K.; Whitten, D.; McLendon, G . J . Phvs. Chem. 1986. 90. 6074. (5) Chandler, R. R.; Coffer, J. L.; Atherton, S.J.; Snowden, P. T. J . Phys. Chem. 1992, 96, 2713. (6) Chandler, R.; Coffer, J. L. J . Phys. Chem. 1991, 95, 4. (7) Fischer, Ch.-H.;Henglein, A. J . Phys. Chem. 1989, 93, 5578. (8) Wan& Y.: Suna. A.: McHuah. J.: Hilinski. E. F.: Lucas. P. A.: Johnson, R . b . J . Chem. Phys. 1990; 92, 6927 (9) Coffer, J. L.; Chandler, R. Mater. Res. SOC.Symp. Proc. 1991, 206, 527. (IO) Coffer, J. L.; Bigham, S.R.; Pinizzotto, R. F.; Yang, H. Nanotechnology, in press. (1 1) Fasman, G. D., Ed.CRC Handbook of Biochemistry and Molecular Biology, 3rd ed.;CRC Press: Cleveland, 1975; Vol. I, pp 589-590. (12) Barton, J. K.; Goldberg, J. M.; Kumar, C. V.; Turro, N. J. J . Am. Chem. SOC.1986, 108, 2083. (13). (a) Turro, N. J. Modern Molecular Photochemistry; BenjaminCummings: Menlo Park, CA, 1978; pp 317-319. (b) Perrin, J. Ann. Chem. Phys. 1932, 17, 283.
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(14) Saenger, W. Principles o/Nuc/eic Acid Structure; Springer-Verlag: New York, 1984. (15) Resch, U.; Eychmuller, A.; Haase, M.; Weller, H. Longmuir 1992, 8, 2215.
(16)Chander,R.R.;Bisham,S.R.;Coffer,J.L.J.Chcm.Educ.,inpieac. (17) Youn, H.-C.;Baral, S.;Fender, J. H. J . Phys. Chem. 1988,92,6320. (18) Henglein, A.; Gutierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983,87,
852.
A Lattice Treatment of Crystalline Solvent-Amorphous Polymer Mixtures on Melting Point Depression Yoshio Hoei,*
-
S&S Japan Co., Ltd., 6- 1 18, Hamazoe-dori, Nagata- ku, Kobe, Hyogo 653, Japan
Kamo Yamaura, and Shuji MaCsuzawa Faculty of Textile Science and Technology, Shimhu University, 3- 15- 1 , Tokida, Ueda, Nagano 386, Japan (Received: July 13, 1992; I n Final Form: October 2, 1992)
The conventional melting (and freezing) point depression equation for solvent crystal-amorphous polymer binary mixtures is reasonably derived using a free energy of fusion which is obtained by modifying Flory’s lattice theory of fusion for semicrystalline polymer-solvent systems. In considerationof the features of small molecular crystallites, a calculation of total configurational entropy is carried out assuming the “extended chain molecular crystal” model. For the heat of mixing, van Laar’s formula is used.
Introduction The classical melting (and freezing) point depression relationships of a crystalline solvent for crystalline solvent-amorphous polymer mixtures are (l/Tm
- l/TOm)(ho/R)
=
-[In (1 - 4)
+ (1 - 1 / 4 4 + x4*1 (1)
and
-
- 1/Tm)(ho/R)
= -[ln (1 - 4) + 4 + X@l (x 0 3 ) (1’) Here T, and To, are the melting temperatures of the mixture and the pure solvent, respectively, ho is the molar heat of fusion, x is the polymer-solvent interaction parameter, x is the ratio of the molar volumes of the polymer and solvent, and $t is the volume fraction of the polymer. These classical relationships has been verified experimentally. These equations have been used to determine x values for a variety of polymer solutions’.* and to examine the departure of swollen polymer gel freezing points from the colligative property) and the effects of mixtures of crystalline species on the phase diagrams and eutectics.e7 On the other hand, the theoretical background for eqs 1 and 1’ was derived from the Flory-Huggins expression of chemical potential of mixing and the condition of phase equilibrium: Le., the chemical potential change of solvent in the crystalline phase is equal to that in the liquid (solution) phase in the equilibrium state.***In this paper, we will attempt to derive a more general expression for eq 1 which reduces eqs 1 and 1’ under certain limiting conditions. The derivation will be performed in terms of the degree of crystallinity and crystallite size on an analogy of Flory’s lattice theory of fusion for semicrystallinepolymer-solvent mixtures9 (1/Tm
Model Our model concept of a single crystallite obeys that of an extended chain molecular crystal where all the molecules are exactly the same size. It has been established that this kind of crystal, for example, one composed of limited molecular weight n-alkanes, is formed.loJ’ To treat the present system in a lattice wherein all the segments of solvent molecules and polymeric *Towhom correspondence should be addressed. 0022-3654/92/2096-10584S03.00/0
structural units are assigned, it is assumed that bundles (i-e,, crystallites) of linear crystalline solvent sequences are of uniform length (tomolecules per sequence) randomly dispersed in the solution (of noncrystallinesolvent and polymer) and each molecule (zo segments per molecule) is regularly placed side by side and end to end within the bundle, as illustrated in Figure la. The structural difference between our model and Flory’s fringed micelle model9 (see Figure 1b) is easily seen by comparison.
Method The parameters used are defined as follows: z is the number of segments of a polymeric structural unit, assuming that the size of a site (cell) forming the lattice is equal to that of a segment of the structural unit and solvent molecule; y is the number of nearest neighbors: No is the number of uniform solvent molecules; N is the number of uniform polymer molecules; x is the number of structural units per polymer molecule; is the total number of crystallite sequences; and wo = mto/Nois the degree of crystallinity. First of all, we estimate a total configurational entropy S, on the basis of the model structure in Figure la, which consists of four different entropies as shown by Flory? If we suppose a single long liiear chain randomly connected with all of the polymer and solvent molecules, the total number of possible arrangements of all the molecules in the chain W,is W,= (No + N)!/N,$N!, which yields the first entropy Next, the number of configurations for the chain must be -timated in the lattice which is comprised of (N,,zo + XZN)sites. In the amorphous region each segment can be succwiively placed, one by one, at any of (y - 1) neighboring sites; thus, it is possible to conveniently express the entropy contribution by employing the disorientation entropy per segment k In [(y - l)/e] as previously formulated in the Flory-Huggins polymer solution treatment.* In the crystalline region the first site of each sequence is occupied by a segment which is followed by a segment which still belongs to the amorphous region, and thus the fmt segment gains entropy. However, no choice of location can be taken for the next (t,,zo - 1) segments which occupy the second and subsequent sites until reaching the opposite end site. A segment adjacent to an oppaeite end one regains entropy upon entering the amorphous region. 0 1992 American Chemical Society
