ARTICLES

Exciton Energy Transfer in Self-Assembled Quantum Dots on Bioengineered BacterialFlagella Nanotubes

Mudalige Thilak Kumara, † Brian C. Tripp,* ,‡ and Subra Muralidharan* ,†

Department of Chemistry and Nanotechnology Research and Computation Center, Western MichiganUniVersity, Mailstop 5413, 1903 West Michigan AVenue, Kalamazoo, Michigan 49008-5410, and Department ofBiological Sciences, Department of Chemistry and Nanotechnology Research and Computation Center, WesternMichigan UniVersity, Mailstop 5410, 1903 West Michigan AVenue, Kalamazoo, Michigan 49008-5410

ReceiVed: NoVember 11, 2006; In Final Form: February 9, 2007

The bacterial flagellin fusion protein, FliTrx, was modified by inserting a peptide containing 24 histidineresidues and shown to successfully export and self-assemble to form flagella nanotubes. The flagella of this“His-loop” protein were harvested and employed as scaffolds for the self-assembly of 3 nm ZnS/Mn (ZnSquantum dots doped with Mn2+ ions) and CdTe quantum dots (QDs). The QDs were shown to form anordered array by transmission electron microscopy (TEM) and were separated by an average distance of 2.4nm. The absorption and emission spectra of uncapped ZnS/Mn QDs exhibited no change upon self-assemblyon the His-loop flagella compared to those in solution. Band gap excitation and subsequent Mn-centeredemission were insensitive to self-assembly on the flagella nanotubes. By contrast, theL-cysteine-capped CdTeQDs exhibited a red shift in both the absorption and emission spectra upon binding to the His-loop flagellacompared to those of the free QDs in solution. This shift was consistent with exciton energy transfer fromsmaller to larger CdTe QDs on the flagella nanotube, indicating that the self-assembly facilitated such anenergy transfer. The 5 nm (18.4 meV) shift in the emission spectrum was similar to previous observationswith organically linked CdTe QDs and theoretical calculations reported in the literature.

Introduction

Biological molecules such as peptides, proteins, lipids, andDNA are being investigated as templates and scaffolds for theassembly and biomineralization of semiconductor nanoparticlesand nanowires. These bio-inspired approaches are interestingbecause they employ a bottom-up approach to manufacturingwhich has certain advantages, including reproducibility at themolecular level, selectivity, for example, high affinity for certainions or small molecules and not for others, and the ability ofbiomaterials to direct crystal morphology in biomineralization;these approaches could surpass current lithographic capabilities.1-5

Tunable optical properties, photostability, relatively high quan-tum yields, and the ability for doping with other elements makesemiconductor QDs increasingly useful in various applications,such as imaging, sensors, lasers, light emitting diodes, solar cells,photon guides, and molecular electronics.6-20 The combinationof biomineralization and/or assembly of semiconductor materialson engineered biological scaffolds and templates can yield novelbottom-up nanoscale systems with unique properties that canaddress limitations of current top-down approaches to manu-facturing electronic devices.

Bacterial flagella are an example of a self-assembling proteinnanotube system which can, in vitro, form nanotubes oftheoretically unlimited length. They are narrow,∼20 nmdiameter, helical protein structures, up to 10-15 µm in length,that extend out from the surface of bacterial cells. They areattached to a short, universal joint-like hook structure that isattached to and rotated by a membrane-bound rotary motorcomplex. The majority of the bacterial flagella nanotube iscomposed of up to 20,000-30,000 monomers of a globularflagellin protein (e.g., FliC), which self-assembles to form ahollow helical structure with 11 units per turn; the distal end,furthest from the cell, is capped by a pentamer complex of theHap2/FliD protein.21-23 Recent X-ray crystallography andcryoelectron microscopy studies indicate that the modelSal-monella typhimuriumFliC flagellin protein consists of fourmajor domains termed D0, D1, D2, and D3.24,25 The largelyR-helical D0 and D1 domains form the core of the flagellananotube and are essential for self-assembly. The solvent-exposed outer D2 and D3 domains are not essential for self-assembly and can tolerate mutation, deletion, and insertions.25

The distance between adjacent D3 domains in the highly orderedflagella structure is estimated to be about 5.4 nm; any mutation,deletion, or insertion of amino acids or chemical modificationof the existing residues on the D2-D3 domain region will havethe same periodicity of spacing.25

Our research program is focused on gaining a fundamentalunderstanding of protein nanotubes derived from bacterial

* To whom correspondence should be addressed. E-mail: subra.murali@wmich.edu; brian.tripp@wmich.edu.

† Department of Chemistry and Nanotechnology Research and Computa-tion Center.

‡ Department of Biological Sciences, Department of Chemistry andNanotechnology Research and Computation Center.

5276 J. Phys. Chem. C2007,111,5276-5280

10.1021/jp067479n CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/17/2007

flagella26 and QDs doped with magnetic nuclei.27-29 An integralpart of these efforts is the investigation of flagella-quantumdot nanocomposites. In this work, the two-dimensional assemblyof ZnS/Mn and CdTe QDs on the surface of bioengineeredflagella and their exciton energy transfer (EET) are presented.We recently demonstrated assembly and manipulation in anoptical trap of bioengineered flagella protein nanotubes withcysteine loops,30 biomineralization of transition metals31 silica,titania, and apatite, and application of flagella as a template forpolyaniline nanowires.26 In these studies, we used the FliTrxfusion protein, composed ofEscherichia coliflagellin (FliC)and thioredoxin (TrxA), where sections of the FliC D2 and D3domains were replaced with thioredoxin. Thioredoxin, in turn,has a disulfide bond-constrained active-site region on the surfaceof the protein that is suitable for insertion and display of looppeptides.32 As previously described, a DNA cassette encodinga loop peptide of twenty one amino acid residues was removedfrom the FliTrx thioredoxin domain, and a new DNA cassetteencoding histidine (His) residues with imidazole side-chaingroups, the seven amino acid “His-loop” peptide (Gly-His-His-His-His-His-His; GHHHHHH), was inserted usingstandard molecular biology methods. FliTrx variants containinginserts with 1-6 repeats of the His-loop peptide were isolatedand characterized with respect to their intracellular proteinexpression and competence for extracellular export and assemblyinto flagella nanotubes. Although expression levels were highfor all six unique FliTrx His-loop variants, export and assemblyof His-loop variants with five and six inserts was very low.Therefore, the His-loop variant with the largest loop peptidethat was still efficiently exported and assembled into flagellananotubes was used in this study, the 4X insert variant with 24histidine residues (GHHHHHH-GHHHHHH-GHHHHHH-GHHHHHH). The FliC protein does not have any tryptophan(Trp) residues, while the inserted thioredoxin protein has twoTrp residues, Trp28 and Trp31, in native thioredoxin, corre-sponding to residues Trp272 and Trp275 in the FliTrx protein.These two Trp residues are located in the thioredoxin active-site region near the base of the peptide loop. These two Trpresidues were replaced with tyrosine residues by site-directedmutagenesis (Trp272f Tyr, Trp275 f Tyr) to minimizepossible absorbance and excitation of the Trp indole ring sidechain when attached ZnS QDs were excited at 300 nm.

Materials and Methods

Protein expression, purification, and self-assembly of the 4Xrepeat His-loop FliTrx variant into flagella protein nanotubeswas carried out as previously described.26,30 Synthesis of Mn-doped ZnS nanoparticles was carried out as described in detailelsewhere, and the significant aspects are provided here. A 45mL volume of HPLC-grade ethanol was bubbled with nitrogenfor 15 min and 990µL of 1 M zinc acetate and 10µL ofmanganese(II) acetate were added to the ethanol.33 A 1 mLvolume of 1 M sodium sulfide solution was added dropwise,and the mixture was stirred for 5 min. The resulting ZnS/Mnnanoparticles that formed were washed with nitrogen-saturatedethanol 3 times, isolated by centrifugation for 10 min at 5000g,and suspended in deionized (DI) water for assembly on flagella.A calculated volume of ZnS/Mn quantum dots to yield anabsorbance value of 0.2 at 300 nm with a Lambda 20spectrophotometer (PerkinElmer, Wellesley, MA) was mixedwith 500 µL of 1 mg/mL flagella solution in 100 mM NaCl,made up to 1 mL with DI water, and equilibrated for 30 min atroom temperature. The fluorescence lifetime and emissionintensity were measured for ZnS/Mn QDs with excitation at

300 nm and emission at 590 nm on an FLS 920 spectropho-tometer (Edinburgh Instruments, Livingston, UK) using a 395nm cutoff filter. A 10 µL volume of this solution was placedon a carbon-coated Formvar/400 mesh transmission electronmicroscopy (TEM) grid for 1 min, and excess water wasremoved by blotting with a piece of filter paper. The CdTequantum dots capped withL-cysteine were synthesized asdescribed elsewhere.34 A 2 mg quantity of nanoparticles cappedwith L-cysteine was dispersed in a 2 mLvolume of water bysonicating for 30 min, and the absorbance of the nanoparticlesat 400 nm was measured. A volume of nanoparticles calculatedto yield an absorbance value of 0.2 at 400 nm was mixed witha mixture of 500µL of 1 mg/mL flagella in 100 mM NaCl, 20µL of 1 M borate buffer (pH 8.2), and 50µL of 1 M NaCl, andthe volume was adjusted to 1 mL with DI water. Absorbancespectra were measured for the appropriate blank solution withoutQDs, and fluorescence spectra were recorded with excitationat 400 nm. The fluorescence lifetime for CdTe nanoparticleswas measured with excitation at 400 nm and emission at 580nm. A 10µL volume of this solution was placed on a carbon-coated Formvar/400 mesh TEM grid for 1 min, and excesssolvent was removed by blotting with filter paper. The averagelifetime for the entire decay process was calculated with FASTsoftware (Edinburgh Instruments, Livingston, UK) using re-convolution fit with instrumental response.

The ZnS nanoparticles were also synthesized by immobilizingHis-loop flagella on a holey carbon TEM grid. For synthesis ofZnS QDs on flagella, a 10µL volume of 0.25 mg/mL flagellasolution was placed on a holey carbon grid, and excess solventwas removed by blotting with filter paper. A 5µL volume of 5mM ZnSO4 was placed on the flagella-coated TEM grid, and 5µL of a 5 mM Na2S solution was placed on the other side ofthe TEM grid. After 5 min, the excess solution was removedby blotting with filter paper. Typical TEM images of previouslysynthesized ZnS/Mn QDs immobilized on the His-loop flagellascaffold are shown in Figure 1a, and a TEM image of ZnS QDssynthesized on the His-loop flagella template is shown in Figure1b. The ZnS QDs nanoparticles synthesized on the flagellascaffold yielded an electron diffraction pattern indicative ofWurtzite structure (Figure 1b inset). In contrast, control experi-ments performed using FliTrx flagella without any insertedhistidine loop peptides on the thioredoxin domain did not resultin formation of any QDs under the same conditions. Duringthe synthesis of QDs on flagella, the high local concentrationof Zn2+ ions, which are complexed to the imidazole groups ofa His residue on the flagella surface, may initiate the nucleationprocess and generate nanoparticles on flagella scaffolds. Figure1c shows the assembly of cysteine-capped CdTe QDs on His-loop flagella. In the case of assembly of QDs on His-loopflagella, the imidazole groups of histidine residues most likelyplay a critical role through coordination with Zn(II) and Cd(II)ions of the QD nanocrystals.

Results and Discussion

The histidine loops on the flagella are present on the D3domain of the flagella nanotube, where each segment is a helicalself-assembly of 11 flagellin protein monomers. This results ina distance of about 5.4 nm between consecutive D3 domains.25

The procedures for synthesizing ZnS/Mn and CdTe nanopar-ticles were optimized to obtain QDs with an average size of 3nm and a polydispersity of 10%. The ZnS/Mn QDs did not havea capping agent, while the CdTe QDs were capped withL-cysteine. The cooperative binding of the peptide loop histidineligands displaced the cysteine ligands from the CdTe surface

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to facilitate their binding to the flagella.10 The solvent-exposedsurfaces of the flagella-bound CdTe QDs most likely are stillcovered with cysteine. Thus, in effect, the CdTe QDs bound tothe flagella nanotubes are coordinated to both histidine andcysteine ligands. The organized assembly of 3 nm (average size)QDs on flagella nanotubes shown in Figure 1c are, on average,separated by 2.4 nm. This separation distance is much largerthan the corresponding distances typically encountered in QDand nanoparticle systems linked with organic molecules.35 TheCdTe QDs have band gaps that are sensitive to their size;excitation with photons in their band gap yields excitons whichemit at wavelengths characteristic of the band gap.35,36 Bycontrast, the fluorescence emission maximum from the ZnS/Mn QDs that is centered at 590 nm is due to the dopant Mn-(II), corresponding to the transition from4T1 to 6A1. Thisemission is due to energy transfer from the exciton to the dopant,and its position and intensity are not very sensitive to the sizeof the QD. However, the emission is sensitive to the concentra-tion and location of the dopant, namely, surface versus latticedoping.27,37 The CdTe QDs, in addition to possessing size-dependent band gaps and emission wavelengths for excitons,are also capable of undergoing interparticle exciton energytransfer and electronic coupling when they are closelyassembled.35,36,38-41 The exciton energy transfer can occur overlonger ranges for various size nanoparticles, while the electroniccoupling, which requires an overlap of electron wave functions,typically occurs for small, organically linked or electrostaticallyinteracting QDs over a short range. Similar processes have alsobeen observed for CdSe and InP QDs and QD-metal nano-particle composites.40,42-47 The QDs self-assembled on thehistidine loops of flagella provide an excellent system to exploreexciton energy transfer and electronic coupling for thesesystems. We have investigated the absorption and emissionspectra and emission lifetimes of ZnS/Mn and CdTe QDs self-assembled on the His-loop flagella to determine if interparticleinteraction leading to exciton energy transfer or electroniccoupling could be observed.

The normalized absorbance and emission spectra of ZnS/Mnnanoparticles assembled on His-loop flagella are shown inFigure 2. The absorption and emission spectra of ZnS/Mn QDsdid not exhibit any change between the QDs suspended insolution and those bound to His-loop flagella. The luminescencedecay of Mn2+ showed biexponential decay, with emissionlifetimes of 2.00 and 0.28 ms, which did not change betweenthe free and bound QDs. The longer emission lifetime was due

to Mn2+ ions in slightly distorted tetrahedral sites in the crystallattice, while the shorter emission lifetime was due to Mn2+

ions adsorbed on the surface of the quantum dots.37 The metal-centered emission was insensitive to binding by histidine andlong-range interparticle interactions on the His-loop flagella.We have observed emission intensity and lifetime changes forthe ZnS/Mn QDs when capped withL-cysteine, which bindsmuch more strongly than does histidine.28

A different behavior was observed for the CdTe QDs on theHis-loop flagella compared to that of CdTe QDs in solution.The normalized absorbance spectrum for the CdTe QDs on His-loop flagella, as seen in Figure 3, showed an increase inabsorbance and a slight red shift, compared to that of the CdTeQDs in solution, which indicates possible electronic couplingbetween closely packed CdTe QDs on the His-loop flagella.The emission spectrum of CdTe QDs on His-loop flagellashowed a reduction in their fluorescence intensity and a 5 nmred shift from 578 to 583 nm, as seen in Figure 4, compared tothat of CdTe QDs in solution. The spectrum for CdTe QDs onthe flagella exhibited a 5 nm redshift corresponding to an energydifference of 18.4 meV between the free QDs in solution andthose bound to the flagella. This corresponds to a change inthe exciton energy level for the flagella-bound QDs which couldbe due to exciton energy transfer from the smaller (larger bandgap) to the larger (smaller band gap) QDs bound to the flagella,given the 10% polydispersity in their size. Such a transfer isfacilitated on the flagella due to the organized self-assembly ofthe QDs, while in solution, the random orientation of the various

Figure 1. (a) TEM image of ZnS/Mn nanoparticles immobilized on a histidine loop flagella scaffold (scale bar 50 nm). (b) TEM image of ZnSnanoparticles synthesized on a flagella scaffold (scale bar 100 nm); (inset) electron diffraction pattern indicating Wurtzite structure. (c) TEM imageof CdTe nanoparticles immobilized on a histidine loop flagella scaffold (scale 100 nm); (inset) high-resolution TEM image of CdTe nanoparticlesimmobilized on a flagella template (scale bar 5 nm). The quantum dots had an average size of 3 nm, and TEM images were recorded with a JEOL1230 TEM instrument operating at 80 kV (a and b) and a JEOL 3011 TEM instrument operating at 300 kV (c).

Figure 2. Normalized absorbance (a) and emission (b) spectra of ZnS/Mn QD nanoparticles self-assembled on His-loop flagella nanotubes.Emission spectra were recorded with excitation at 300 nm.

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QDs results in an average emission spectrum. The emission peakwidth at half peak maximum for the CdTe nanoparticles on theflagella scaffold was 52 nm compared to 55.5 nm for that ofthe QDs in solution, which is also indicative of the excitonenergy transfer.

These observations are similar to those of Meijerink for CdTeQDs linked by thiol ligands and theoretical calculations ofForchel for exciton energy transfer between CdTe QDs as afunction of their separation.35,36 The 18.4 meV energy changeis consistent with the average interparticle distance of 2.4 nmfor the CdTe QDs on the His-loop flagella. Further evidencefor the exciton energy transfer among CdTe quantum dots isalso obtained from emission lifetime measurements by monitor-ing the emission intensity as a function of time as shown inFigure 5. The exciton lifetime of the cysteine-capped nanopar-ticles of CdTe QDs in solution was determined to be 18.1 ns,and those for the CdTe QDs on His-loop flagella were foundto be 15.5 ns. The difference in the lifetimes of the free andflagella-bound nanoparticles is evident from the inset in Figure5 displaying the emission intensity for the initial 30 ns. Theapproximately 2.5 ns reduction in the lifetime of the CdTeexcitation can be rationalized as arising from the exciton energytransfer from small to large flagella-bound CdTe quantum dots.

Conclusions

We have successfully self-assembled ZnS/Mn and CdTe QDson flagella nanotubes displaying histidine peptide loops on theirouter domain regions. The interparticle interaction of the QDsseparated by about 2.4 nm does not lead to a shift in the emissionwavelength maximum nor a lifetime change for the Mn-centeredemission of the ZnS/Mn QDs. Such an interaction for CdTeQDs resulted in exciton energy transfer from smaller to largerQDs, resulting in a 5 nm redshift (18.4 meV energy) of theexciton emission peak and a shortening of the exciton lifetimeby about 2.5 ns. The flagella-quantum dot composites arefundamentally interesting and important bionanostructures, andfurther investigations with other engineered peptide loops areunderway.

Acknowledgment. This research was supported by the W.M. Keck Foundation and the Nanotechnology Research andComputation Center at Western Michigan University.

Supporting Information Available: Information aboutDNA primers used in FliTrx site-directed mutagenesis, the His-loop flagellin amino acid sequence, TEM images of parts ofbacteria with flagella and pili, purified flagella, AFM imagesof flagella on glass, additional TEM images of QDs on flagella,and fluorescence decay curves of CdTe QDs, CdTe QDs onflagella, and ZnS/Mn QDs on flagella. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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Figure 4. Emission spectra of CdTe quantum dots (a) in solution and(b) self-assembled on flagella. Inset shows emission spectra normalizedfor absorbance at the excitation wavelength of 400 nm.

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