Characterization of quantum(cid:173)
`confined CdS nanocrystallites
`stabilized by deoxyribonucleic
`acid (DNA}
`
`Jeffery L Coffert§, Shelli R Bighamt. Russell F PinizzottoU
`and Hong Yangi
`tDepartment of Chemistry, Texas Christian University, Fort Worth, TX 76129, USA
`tCenter for Matl!rials Characterization. University of North Texas. Denton.
`TX 76203. USA
`
`Received 30 January 1 992. accepted for publication 26 June 1992
`
`Abstract. The biopalymer calf thymus deoxyiibonucleic acid (DNA) is emoloved
`to stabilize cadmium sulfide crvsr.lliUIS in the quantum confinement sae revime
`(U·CdS). tn tn1s work. the synthells and cti.ncteriZatloo of tti.e semiconductor
`'quantum .dots' 1s described. Ti- Q.Cds clustttS .,. easily ptepared in aqueous
`solution at room temperature and are exnen.ly stable (for more than 17 months
`when stored at 5"C). High·resolutlon transmission electron microscopy shows
`that the crystallites have an average cfiameter of 5.6nm, with lattice images and
`diffraction pattems consistent with the zinc·blende structure of CdS. For
`approximately 15% of the particles, unique hollow·sphere· or hollow-hemisphere·
`shaped CdS structures are obseNed. and their pteSence attributed to the influence
`of the ONA host. Spectroscopically, these clusters show an absorption edge blue·
`shifted from that of the bulk, consistent with quantum confinement. and broad
`trap emission cheractllfistic of an appreciable number of defect sites at the
`semiconductor cluster interface, apparently induced in pert by the host
`polynucleotide. The effects of the Q.CdS clusters on the macroscopic properties
`of the DNA are illustrated by the change in intrinsic viscosity upon addition of
`cadmium ions and subsequent CdS formation.
`
`(
`
`I I
`
`1. Introduction
`
`The study of semiconductor nanocrystallites with a
`panicle diameter comparable to the size of the bulk
`exciton, so-called 'quantum dots', is of current interest
`from the diverse perspectives o( chemistry, physics, and
`materials science [ 1-4]. Their size-dependent bandgap.
`incomplete band structure, and three-dimensional con·
`finement of charae carriers give rise to unique photo·
`physical properties (S-7]. Several preparative routes for
`the synthesis of these materials are currently employed.
`Each utilizes macromolecular stabilizers or surface re(cid:173)
`actions that inhibit particle growth of the crystallites.
`faamples of such 'arrested precipitation' reactions in·
`elude the use of simple polymers [8-11), inverse micelles
`[12), organic capping reagents [13-15), zeolites (16, 17],
`biosyn thesis [ 18, 19], Langmuir-Blodgett films [20],
`layered phosphonates [21), and lipid bilayer membranes
`[22).
`The use of polynucleotides for semiconductor cluster
`stabilization is appealing for two main reasons. First.
`
`!Authors to who,, 1ny ce<rnpondenct should bt sont.
`
`0957.448~ 192 1020069 + 08 SOHO © 1992 IOP P·Jbl11h1"!l l td
`
`there is an extensive body of knowledge regarding
`nucleotide structure and dynamics in solution from both
`macro· and microscopic . techniques (23]. Second, a
`useful feature of nucleotides as polymeric stabilizers is the
`possible control of nucleotide composition as well as
`three-dimensional structure (single·, double·, or triple ..
`helix) at the nanoscale level An area of fruitful research
`would be to exploit this knowledge or polynucleotides as
`a means of examining the structural inlluencc of tbest
`stabilizers on semiconductor cluster formation. Transi ..
`tion metal ions, in gcntn~ can bind to nucleic acids at
`three possible locations [24]: (i) the anionic oxygen
`atoms or the outer phosphate groups; (ii) at the hydroxyl
`sroups of the ribose .~ugar moieties; and (iii) at the
`nitrop:n 1to111S of the purine and pyrimidine bases. Sine~
`these semiconductor particles of cadmium sulfide are
`prepared by initial metal ion addition to the DNA,
`followcJ by the introduction or sulfide, there exist several
`possible spatial regions or the DNA where the polynu ..
`cleotide should possess an affinity for the semiconductor
`particle.
`We initiated our studies with an examinadon or the
`ability of calf lhymus deoxyribonucleic acid to stabiliz1:
`
`6$
`
`1 of 8
`
`Nanoco Technologies, Ltd
`EXHIBIT 1013
`
`

`

`semiconductor cluster formation and
`to control
`the
`resultant cluster properties. Calf thymus DNA has often
`been chosen in the past to probe interactions of trans-
`ition metal species with nudeic acids [25]. We report
`here the details of a synthetic route to quantum-confined
`cadmium sulfide (Q-CdS) stabilized by DNA obtained
`from calf thymus, and characterization of the Q-Cc1S by
`high-resolution transmission electron microscopy (HREM)
`and absorption and emission spectroscopy (steady-state
`and time-resolved). At the macroscopic level, the effect of
`US5 cluster formation on the intrinsic viscosity of the
`DNA is also reported.
`
`2. Experiment
`
`2.1. Preparation of Q-CdS/Culf thyMOS DNA
`In a typical experiment4 freshly prepared Cd(Cl0J 2 -
`6H120 (5.0 A1 of a I M aqueous solution, prepared from
`the solid; Johnson-Matthey, electronic gradie) was
`diluted to 2 ml and purged thoroughly (for about 20 min)
`with nitrogen (final Cd" concentration 4x 104"M). In
`a separate flask, fresh Na 2S (5.00p or a I M solution,
`prepared fronm the solid; 98Y. Aldrich) was diluted to 5 ml
`and also purged thoroughly with nitrogen (final S2 -
`concentration 4 x l0- M). In a third flask, approximate-
`ly 15mIg Calf thymus DNA was dissolved in 5.0 ml distilled
`decionized H120 in a 5Omi round-bottomed flask. The
`nucleotidle is slow to dissolve and should be allowed to
`stand for about 30mim, whereupon it can then be
`thoroughly mixed. The relative molar concentration of
`nucleoticie was determined spectrophotometrically by
`employing an e-value of 6600M-'cm-' 'for the DNA at
`;2260 [26]. The nucleotide flask was fitted with a septum
`and purged very slowly with nitrogen for about 20min.
`To form Q-CdS clusters, the 2m! Cd2 * solution was
`added to the nucleotidle, and the mixture was purged with
`nitrogen for another 5min. The 5i S
`solution was
`then transferred
`to the reaction flask containing the
`nucleotide and Cd2 + via syringe-, the near-instantaneous
`appearance of a yellow color was observed.
`
`2.2. Transmission electron microscopy
`High-resolution transmission electron microscopy was
`performed at the Center for Materials Characterization
`of the University of North Texas using a Hitachi H-9000
`operating at 3C0 kcy. Samples were prepared by concen-
`trating one of the above CdS/DN.A samples in a centrifuge
`by spinning at 420009 for 20min. The denser CdS/DNA
`material was collected by pipet and 1-2 drop aliquots
`were allowed to air-dry on amorphous carbon films
`supported by standard copper TMm grids. HRBi images
`were obtained using an objective lens aperture that
`allowed all diffracted beams with a d-spacing larger than
`(400) to contrib~ute to the images. Selected-area electron
`diffraction pattern (SADP) analysis was used to measure
`the interplanar spacings.
`
`2-1 Steady-state spectroscopy
`Absorption measurements were made using either an HP
`8452A diode array spectrophotometer or a Van i
`ay
`3 double-beam instrument. Steady-state photolumine,
`cence (Pt) measurements were recorded using ,a spcx.
`Fluorolog-2 0.22 mn double spectrometer. Excitation was 12
`provided via tight from a 450 W Xe lamp focused into a
`single 022 mn monochromator, the typical excitation
`wavelength was 375 am. Emission spectra were corrected
`for fluctuations in photomultipbier tube response
`
`2.4. Viscosity mueasunsnents
`Viscosity measurements were obtained with a conven-
`tional Ostwaldi-type
`viscometer
`and a constant-
`temperature bath (±0.&Cl.
`
`2.5. Tlime-resolved spectroscopy
`For these experiments, a setup at the Center for Fast
`Kinetics Resecarch at the University of Texas-Austin was
`used. A Coherent Antares Nd:YAG lase, mode-locked at
`76 MHz and firequency-doubled
`to 532nt with a trP
`crystal, produced a train of pulses 131 n apart with a
`pulsewidth of 70 us and an average power of 2W. This
`was used to pump a home-built dye laser and cavity
`dumper combination containing Rhodamine 6G. The
`dye laser was tuned to 614nni, and a Cannac Systems,
`1=, Brag cell driver was adjusted to produce a trinm of-
`pulso ca 6ps wide at a repetition rate of 1.9 MHzrorn
`'were 11he
`the cavity dumper. The dye laser pulses
`frequency-doubled
`to 307 am with a rco
`crystal *,to'
`provide the excitation source uv power never Ox edeI
`I mWainf'. Prior to frequency dloublingx a pelickle
`beampiitter led about 2% of the dye beam to a photo-
`diode, which provided the start pulse Fluorescefice
`photons were detected perpendicular to the excitation by
`a Hamamatsu R2809J-07 microchannel plate, which
`provided the stop pulses. Wavelength selection for tine
`fluorescence was achieved by placing an appropriate
`narrow-bandpass
`filter between the sample and
`the
`microchannel plate. Start and stop pulses were amplified
`by a Phillips Scientific Model 774 ampliffier before being
`passed to a Tennelec: TC 454 constant-fraction discrim-
`nator. The discriminator outputs were led to the start
`and stop channels of an Ortec 457 time-to-amplitudit
`converter, whose output was passed to a Tracor Nor-
`themn TN 7200 multichannel analyser (mc:A). Data from
`the mcA were collected by an IBM-compatible personal
`computer for data analysis, storage, and display.
`
`3. Results and discussion
`
`3.1. Synthesis/particle stability
`In this work, we have employed DNA obtained from call'
`thymus to stabilize the formation of CdS aystallites in
`the size domain exhibiting quantum confinement (Q-
`CdS). Tile synthetic methodology required is straightfor-
`
`2 of 8
`
`

`

`(
`
`ward and carried out in two steps: First, a sub-millimolar
`(10' M) aqueous solution of cadmium ions is added to a
`solution o1' DNA of about to-3 M in nucleotide concen-
`tration. Alter
`thorough ebullition with nitrogen, a
`10O M solution of sulfide is then added to this mixture
`to generate the desired CdS cluster
`10' M Cci'(aq) + 10-' M DNA-* Cdl'/D.sA
`
`Q-Cd S/ON A
`
`The intense yellow color that appears instantaneously is
`clearly consistent with the formation of cadmium sulfide.
`These clusters are stable from flocculation for 1-2 weeks
`when stored at room temperature in the open air; if
`somewhat more judicious care is used for their storage
`the CdS
`(i.e., if they are stored in closed vials at 5'C)
`particles are stable for greater than 17 months, with no
`evidence of aggregation or formation of bulk polycrystal-
`line material. It is also clear that a polymeric nucleotide is
`required for cluster stability, since analogous experi-
`ments carried out with the monomeric nucleotides aden-
`osine triphosphate (ATm) or adenosine muonophosphate
`(AMP) at identical concentrations/conditions to the DNA
`experiments result in CdS material that flocculates within
`
`12-24h,
`regardless of storage conditions
`(N2 at-
`rnosphere, temperatures ranging from !OC to - 60'C).
`
`1.2. TEN characterizadion
`A typical transmission electron micrograph of a Q
`Cd/DNA sample is presented in figure 1. The lattice
`planes of numerous crystalline particles are observed.
`The observed lattice spacings are consistent with the
`zinc-blende phase of cadmium sulfide. Particle size dlistri-
`bution analysis was performed by measuring several
`hundred individual particle diameters using a Houston
`Instruments Digitizing Tablet interfaced to an IBM-PC
`A WAIC program calculates both normal and log-normal
`distribution statistics and histograms. The statistical data
`are presented in table 1. The avernage particle sizie was
`5.6mar, with a maximum of l2mm and minimum of
`23nam. The normal and log-normal histograms are
`presented in figure 2. From these graphs, it is apparent
`that the particle size distribution is best described using
`log-normal statistics.
`Confirmation of the Q-CdS as CdS with the zinc-
`blendle structure was obtained using sADP&s Figure 3
`shows a typical pattern. The calculated d-spacings along
`with the literature values for CdS with the zincblende
`
`Figure 1. High-resolution
`thymus ONA.
`
`TEM image of Q-CdS prepared in the presence of calf
`
`Table 1. Particle size distribution data.
`
`Normal distribution analysis
`
`Log-nlormfal distribution analysis
`
`Average particle size
`Standard deviation (s)
`% standard deviation
`Maximum particle size
`Minimum particle size
`
`5.55nm
`1.83nm
`30.98%
`12.1 nm
`2.4 nm
`
`Log average particle size
`Log standard deviation(s)
`
`Log maximum particle size
`Log minimui particle size
`
`0.72
`0.014
`
`1.08
`0.37
`
`3 of 8
`
`

`

`Particle Size Distribution Anslysis
`Linear Gaussian Statistics
`
`25-
`
`Table 2. Comparison of observed selected area diffraction ,t
`(SAO) data for Q-CdS/calf thymus DNA Particles with thgt of
`ICPDS values for zirnc-blend. (ZB) CdS. %vurtzite (W) CdS.
`and graphite.a
`
`Experimental
`d-spocing
`
`ZB-CdS W-CdS
`
`Graphite
`
`0
`
`6
`
`0152
`Particle Sin (Mt)
`
`Particle Size Distribution Analysis
`LogNorinal Distribution Statistics
`
`.. M...
`
`I L
`
`0.6
`.i
`LOG I Particle Size (nm) I
`Figure 2. Particle size distribution for O-CdS particles
`stabilized by calf thymius DNA.
`
`to
`
`Figure 3. SAD pattern for a O-Cd1S particle stabilized byr
`calf thymus DNA.
`
`structure, CUS with the wurtzite structure, and graphite.
`as published in the ICPDS files, are shown in table 2
`[27]. The experimental d-spacings are in complete agree-
`
`'All value ar in angstromu (A). The data correspond to a cubic
`Press with a lattice pa rameter & - 5.7200A.
`
`ment with those of CdS in the zincblende form, with all of
`the diffraction rings present It should bec noted that the
`observed particles cannot be due to small graphitic
`inclusions in the amorphous carbon support films, since
`extra diffraction rings due to graphite are not seen in the
`sourps, none of the observed lattice spacings Jin the
`particles are consistent with graphite, and the cirved
`lattice planes typical of giraphite are not observed 2
`In some cases, the centers of the particles; have
`different contrast than the periphery (fiur 4). inuder
`certain imaging conditions, the center of a particle may
`lack contrast entirely and h4ve a ring or donui shape.
`However, it is usually possible to find objective-lens
`defocusing conditions under which the lattice planes are
`visible across the entire particle, but with reduced con-
`trast in the center. Approximately 15%1 of the particles
`exhibit this effect These images may be due to particles
`shaped like either 'hollow spheres' or 'hollow spheres
`(mushroom caps)'. Natam image calculations will be per-
`formed to test this hypothesis.
`Given the above observations, determination of the
`exact particle structure may provide information on the
`details of the mechanisms of particle nucleation and
`growth. For example, the particles may nucleate and
`gprow while still attached to the DNA chain. Alternatively,
`the particles may nucleate along the biopolymer, detach,
`and then grow later. One possible scenario is that it, is a
`consequence of the polynucleic acid 'template' used. As
`pointed out in section 1, there are three possible sites for
`initial Cd"
`ion/DNA interaction in the preparative
`scheme employed here. Clusters that are formal by
`sulfide addition to Cd 2
`ions bound to the inner base
`pair riegion are somewhat more sterically restricted. The
`vast majority of clusters, however, are quasi-spherical in
`shape, implying formation along the periphery of' the
`phosphate groups.
`
`4 of 8
`
`

`

`Lnaracierwiuuil Qj
`,,.-
`
`-
`
`Figure 4. HREM image demonstrating t unique structure observed for'some 0-
`CdS particles. Lattice fringes fromn other typical sphieuical particles can be seen
`surrounding this unique structure.
`
`3-3 Spectroseopkc properties
`Optical absorption, photoluminescence excitation (El),
`and photoluminescence (PL) spectra for a representative
`sample of Q-CdS prepared at a concentration of
`4 x 10-'M Cd and S and 2.5Sx 10-3M nucleotide are
`illustrated in figure 5.
`The observed absorption threshold at about 480nmn
`is clearly blue-shifted from that of bulk CdS (510 am)
`consistent with retention of quantum confinement in the
`cluster C1U. No distinct higher-energy transitions, such as
`the occasionally observed 1S - is. [28), are detected in
`this particular case. This is due most likely to linewidth
`broadening effects resulting from a distribution of Q-CdS
`particle sizes for this preparation.
`A narrower band can be detected, however, in the
`corresponding PLr spectrum of Q-CdS/calf thymus DNA
`
`700
`
`600
`30 4'00
`500
`00 Wavelength (nm)
`Figure 5. Optical absorption, photoluminescence
`excitation (PLE), and photoluminescence (PQ) spectra for a
`representative O-CdS sample prepared at a concentration
`of 4.2 x10- aM Cd1 and 2.5 x10'-1M nucleotide. PLE
`spectra were monitored at 612 nm, while the FL exciting
`wavelength was 375nrm.
`
`monitored at 612 nm. By monitoring a single emission
`wavelength, a narrower distribution of crystalites is
`excited (as contrasted with the absorption experiment)
`and hence a sharper excitation maximum is observed at
`400nm. This maximum is attributed to the is-is
`'exciton' state.
`Fially, the Pt. spectrum of Q-CdS/brNA, also shown
`in figure 5, reveals broad trap emission ranging from
`480-720nm, with slight dominance of intensity near
`620 am. Earlier studies for Q-CdS have attributed emis-
`sion in the 500-600nm region as cadmiumruiated (VcO)
`[16,29-311 while emission greciter than 600nm is re-
`lated to sulfur vacancies (NV*A a specific type of this defect
`being Cel"
`centers at th;se situs on the particle surface
`[32-35]. For most existing Q-CdS preparations such as
`stablizrs, 1: 1
`those employing
`inverse micelle
`cadmium-to-sulfur ratios typically yield weakly emissive
`samples (12). The presence of such 3trong emission
`observed for 1: 1 Cd/S prepared in the presence of calf
`thymus DNA is significant, and suggests that the host
`polynucteotide is responsible for the particuilar surface
`photophysics observed here.
`In order to probe the chemical origin of these defects
`further, we have examined the effects of varying cadmium
`ion concentration during particle formation while hold-
`ing sulfide concentration constant, and vice 'versa. The
`results are illustrated in 11gure 6. Identical absorption
`spectra are obtained during the course of these experi-
`ments consistent with
`indistinguishable particle size
`distributions. In figure 6(a), the effect of increasingCd
`during initial particle formation results in increased
`emission intensity overall, most notably at 620 nnL Such
`an observation is consistent with the above interpre-
`tation that emission in this region is related to sulfur
`vacancies, since presumably the excess cadmrium ions
`show up as defects at the semiconductor cluster surface (a
`
`5 of 8
`
`

`

`16
`
`20
`
`15500
`
`11590
`
`IS
`
`EW
`
`,
`
`0
`
`4
`
`8
`12
`Time (insec)
`Figure 7. Decay of O-CdS/o,NA emission monitored at
`580 and 640 nm, along with ithe comesponciing fits to the
`data according to equation (1). A typical instrument
`response function is shown as a dashed curve.
`
`700
`
`400
`
`550
`475
`62S
`Wavelength (nmn)
`Figure 6. Emission spectra of O-CdS solutions stabilized
`in 2.SX 10-3M calf thymus DNA, demonistrating (a) the
`affect of varying (C2+] while holding [S'-J constant at
`4.2 x 10O-'M and (b) the effect of varying [S2l while
`holding [Cdl*] constant at 4.2x 10-IM.
`
`phenomenon observed previously for Q-CdS synthesized
`in inverse micelles [12, 13)). Correspondingly, the pre-
`sence of excess sulfide (figure 6(b)) results in a diminution
`region, while
`of emission intensity in the 500-600 ni
`that greater than 600 tan sharpens considerably. The
`presence of the excess sulfide during cluster formation
`appears, then, to interact with Cd atoms responsible for
`the 5S0nm emission. Such behavior is analogous to the
`quenching of 580mnn emission of Q-CdS
`in zeolites
`(16).
`observed previously with the addition of excess H2
`This emission has also been attributed to cadmium atom
`defect sites in the cluster.
`For both types of cluster samples (excess Cd orS)
`emission intensity grows with time. The linewidth of
`emission for Q-CdS prepared with excess sulfide narrows
`most markedly upon aping in a closed vial at P'C (OHM
`shrinks by 35% in 5.5 weeks), suggesting that under these
`particular conditions the semiconductor surface recon-
`structs to yield a slightly narrower range of defect sites
`(traps).
`The nature of these photoexcited charge carriers was
`further examined by monitoring the decay or' the Q-
`in the nanosecond
`CdS/1DNA emission at 580 and 640 rn
`time regime, using time-correlated single-photon count-
`ing techniques. The results of this study are illustrated in
`figure 7. From a visual examination of the decay curves,
`it is clear that the 640 tim emission is considerably sloWQer
`in its decay when compared with the 580nam emission.
`The experimental PL decay Curves were reconvolved with
`the instrument response functions and then fitted to a
`modified version of the Kohlrausch/Williams-Watts
`function, which combines a single-exponential term with
`a stretched -exponential term:
`
`(1:2)
`
`1(t) - A, exp(-t/ 1) + A2 exp(-.r/r 2f
`(1)
`McLendon and co-workers [36) have utilized this func-
`tion, both for its simplicity and its qualitative resam-
`blance to the experimuental data, to measure PL decay in
`Q-CdIS and Q-Cd3A&2. We note that our attempts to use
`a simple double-exponential function were insufficient to
`Yield satisfactory fits of the data obtained herein, which is
`also consistent with previous observations. The para-
`meter Pi(0 <5P < 1) in the stretched-exponential term is
`inversely related
`to the distribution of decay
`times
`present (as Pi becomes smaller, the distribution of decay
`times becomes broader). The parameter it is therefore
`only the maximum in a distribution of decay timne4 and
`the following method of Lindsay and Patterson [37] can
`be used to extract the average decay time <T2> froth the
`stretched-exponential term:
`(2)
`- r,r(P-'YP
`where r is the gamma functi on, and t 2 and P are the
`corresponding parameters in the stretched-exponentia
`term. Mthough the quality of the fit of the 580mn docay
`curve is not as good as that of the corresponding 640 n=
`curve (92 of 2.3 vensus 65), a comparison of the values
`obtained for (T2>
`from the two CUrve fits reveals that
`those of the longer-wavelength emission are nearly an
`order of magnitude greater (table 3). This result is
`consistent with the qualitatively observed slower de.cay
`curve of the 640 ni emission. This observation
`that
`shorter average lictimes occur at shorter wavelengths
`has been observed previously for Q-CdS; it is noted to be
`a characteristic signature of donor-acceptor emisfion
`and reflects a significant coulombic; interaction between
`traps [29.,33). This view of electron hole-pair recoin-
`bination in CdS/DNA~ Cluster materials is consistent with
`Previous studies of Q-CdS emission stabilized by other
`media,
`reflecting simultaneous
`radiative and non-
`radiative tunneling between deeply trapped. carriers with
`strong lattice phonon participation [33]1. It should be
`emphasized, however, that the dominant mechanism
`operative in the charge carrier recomnbination of these
`CdS clusters at room
`temperature
`is clearly non-
`radiative.
`
`6 of 8
`
`

`

`Table 3. Time-resolved
`photoluminescence results
`for Q- CdS Prepared in the
`presence of calf thymus
`DNA'
`
`Characterization of Q-CdS nanocrystallites stabilized by DNA
`
`picture of an intimate CdS ClUSter/DNA
`interaction o1b-
`tained from the microscopic techniques of spectroscopy
`and microscopy.
`
`SBO nm
`
`640 nm
`
`4. Conclusions
`
`r, 0.33
`3.92
`<ri>
`0.23
`0
`8 .5
`
`0.45
`33.3
`0.18
`2.3
`
`aThe parameters listed wer
`ertracted from PL. decay Curves
`using the fitting function given
`in equation (1). AMl r-values are
`in ns. <re) values were
`coiculated using equation (2).
`
`1.4. Cluster effects on DNA VkCsc5sty
`
`A natural Complement to the above studies that analyze
`the effect of the DNA On the semiconductor cluster is just
`the inverse; i.e., probing the effects of the cluster on the
`polynucleotide. In this context, and at the macroscopic
`level, previous measurements have illustrated the ability
`of viscosity to detect gross changes in DNA duplex
`structure. Such changes are typically brought about by
`some sensitive environmental perturbation such as pH,
`nucleotide concentration, or metal ion addition, (38].
`to be made, one must
`For meaningful comparisons
`
`%lP [n] (I - kPoCill)
`
`(3)
`
`of
`dly
`
`where n.,, = (?I - no)pl
`is the molar concentration
`,
`nucleotide (mol I") and k is Hugoits' coefficient (uu
`0.5) (393.
`the
`At molar nucleotide concentrations of 10-3,
`ite
`highly polymelized calf thymus DNA solutions are qu
`As
`viscous, with [ il-values of the order of 5681lmo -'.
`ns
`expected, addition of' a I0-'M solution of Cd
`io
`to
`results in an average lowering of this value by 12.5%
`has
`4971 mol-''. A lowering of polynucleotide viscosity
`Mle
`been observed previously for this and other polariza
`transition: metal ions, and is attributed to metal bind
`primarily to base pairs with some disruptiop' of hydroi
`bonding (i.e., a destabilizing of the helix, reftectinj
`lower axial ratio) [40]. Interestingly, subsequent forr
`tion of Q-CdS clusters by sulfide addition to the ab'
`solution reveals that cluster formation for
`DNA/Cd
`the viscosity [q]) to increase by approximately I10% to
`average value of 548 1 moll -'. We attribute this increa
`viscosity to the nucleoticle strands which now stror
`interact with the semiconductor surface (see abo
`causing a gross distortion of the DNA structure in sol
`as it is influenced by the large (56 A) quasi-spher
`cluster particles. In terms of viscosity, a large increas,
`axial ratio is consistent with a 'balling up' Of the D1,4
`the polynucleotide wraps itself around the cluster surf;
`these macroscopic observations mirror
`Overall,
`
`We have demonstrated a number of significant feature;
`regarding the ability of calf thymus DNA to Stabilize the
`formation of quantum-confined crystalltes of cadmium
`sulfide (Q-CdS). The semiconductor material formed is
`extremely stable under, reasonable storage conditions
`(5*C) High-resolution
`electron microscopy unam--
`biguously demonstrates that the material is of zinc.
`blendle form and of average diameter within The regime of
`quantum confinement, with some unique structural
`features apparently induced by the host pollynticleoticle.
`Absorption spectroscopy confirms this quantum confine.
`mient in terms of the blue shift of its absorption edge;
`emission spectroscopy reveals that the CdS surface is full
`of defects, induced in part by the significant interaction
`with the DNA- The deccay of this trap photoictisinescence
`is consistent with its characterization as donor-acceptor
`emission. A complementary examination of the effects of
`the cluster on the polynucleotide viscosity present a
`picture of a strong CdS/bNA interaction, consistent with
`the other measurements in this work,
`
`Acknowledgments
`
`The authors of this work gratefully acknowledge finan-
`cial support from the Robert A Welch Foundation, as
`well as partial support by the Donors of the Petroleum
`Research Fund, administered by the American Chemical
`Society (JI.Q. Time-resolved emission measurements
`were performed at the Center for Fast Kinetics Research
`(CFKCR), which is supported jointly by the Biomedical
`Research Technology Program of the National Institutes
`of Health (RR00886) and the University of Texas-
`Austin. The expert assistance of Dr Steve Atherton and
`Paul Snowden of the CFKR is gratefully acknowledged
`with these time-resolved measurements. We also thank
`Robin Chandler of TCU for helpful discussions.
`
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