luminescent semiconductor nanocrystals , such as those made of cdse  zns core - shell quantum 
 dots ( qd 's ) , provide substantial advantages for use as stable fluorophores in 
 biological assays and imaging . as synthesized by conventional methods using high - temperature solution reaction 
 from organometallic precursors ,
highly luminescent qd 's are capped with 
 hydrophobic organic ligands primarily made of a mixture of trioctylphosphine / trioctylphosphine 
 oxide ( top / topo ) .
methods reported to date for achieving water solubility of such materials include silica coating , encapsulation of the native top / topo - capped qd 's within amphiphilic polymer shells   or lipid micelles , and cap exchange of the 
 native top / topo caps with hydrophilic ligands [ 46 ] .
the strategy based on cap exchange with 
 bifunctional ligands is relatively simple to implement and has the potential to 
 provide compact hydrophilic qd 's , a desired property in targeted studies , 
 including fluorescence resonance energy transfer- ( fret- ) based sensing and 
 cellular uptake [ 711 ] .
we have previously utilized readily available thioctic acid and polyethylene glycols 
 ( pegs ) in simple esterification schemes , followed by reduction of the 
 1,2-dithiolane to synthesize a series of peg - terminated dihydrolipoic acid 
 ( dhla  peg ) capping substrates .
the cap exchange reaction of top / topo - capped 
 qd 's with these substrates produced water - soluble nanocrystals that are stable 
 over extended periods of time and over a relatively broad ph range , from weakly 
 acidic to basic conditions ( ph 512 ) . 

though compact and stable , those ligands lack specific functional end 
 groups and do not allow easy implementation of simple conjugation techniques , 
 such as avidin - biotin binding . in this study , we further expanded those findings and report the design and synthesis 
 of ligands functionalized with a biotin end group .
the designed ligands have a central 
 tetraethylene glycol ( teg ) segment , a dithiol terminal group for anchoring on 
 the qd surface and a lateral biotin . 

appending biotin at the end of surface - attached ligands should permit 
 the use of the ubiquitous avidin - biotin binding motif to conjugate qd 's to 
 proteins and other biomolecules via an avidin bridge .
cap exchange reactions were carried out with 
 mixed ligands and preliminary binding assays of the biotin - coated water - soluble 
 qd 's to neutravidin - functionalized substrates showed that specific capture of 
 the qd 's due to avidin - biotin interactions was achieved .
all manipulations were carried out under dry nitrogen and air - sensitive solids were handled in an mbraun labmaster 
 130 glovebox .
triphenylphosphine , thioctic acid , 4-(n , n - dimethylamino)pyridine , n , n-dicyclohexylcarbodiimide , and n - hydroxysuccinimide were purchased from acros organics ( morris plains , nj , usa ) .
sodium azide and biotin were purchased from 
 alfa aesar ( ward hill , mass , usa ) .
methanesulfonyl chloride was purchased from gfs chemicals ( powell , ohio , 
 usa ) .
sodium borohydride was purchased from strem chemicals ( newburyport , mass , usa ) . 

all the other chemicals ( including solvents ) were purchased from sigma - aldrich and acros organics . 

chemical shifts for h nmr spectra are reported relative to tetramethylsilane ( tms ) signal in the deuterated solvent ( tms ,  = 0.00 ppm ) .
column chromatography was performed on bench top , using silica gel ( bodman industries , aston , pa , usa , 60   , 230400  mesh ) .
electronic absorption spectra were recorded using an hp 8453 diode array spectrophotometer ( agilent technologies , santa 
 clara , calif , usa ) , while fluorescence spectra were collected using a spex fluorolog-3 spectrophotometer ( jobin yvon inc , edison , nj , usa ) . to account for the nonlinear ( wavelength - dependent ) quantum efficiency of the pmt detector ,
the fluorescence 
 spectra were corrected using calibration curves accounting for the wavelength - dependence of the pmt 's detection efficiency .
the choice of a short teg 
 segment to test this synthetic scheme was motivated by the well - defined chain length of the teg molecules compared to longer peg chains , which is expected to make separation using column chromatography easier . in the following section we detail the 
 synthesis of each intermediate compound necessary for preparation of the final dhla  teg 
all the compounds were characterized by thin layer chromatography ( tlc ) and h nmr .
diazide - functionalization of teg ( 3 ) : 
 compound 3 was prepared 
 following published procedures .
teg ( 40.0  g , 206  mmol ) , tetrahydrofuran ( thf ) ( 350  ml ) , and methanesulfonyl chloride ( 53.0  g , 463  mmol ) were mixed in a 1  l round bottom flask and cooled to 0c .
a solution of 
 triethylamine ( 49.0  g , 484  mmol ) in thf ( 50  ml ) was added drop - wise over 30 
 minutes and the mixture was stirred at room temperature over 20 hours .
the reaction was then diluted with water ( 200  ml ) and nahco3 ( 12.5  g ) . 

sodium azide ( 36.0  g , 554  mmol ) was added and the biphasic reaction 
 mixture was first heated to 65c to distill off the thf , and then to 70c for 
 14 hours .
the reaction mixture was 
 cooled to room temperature and extracted with ether ( 3  75 ml ) .
the combined organic extracts were dried over 
 mgso4 , filtered , and evaporated to give brownish oil .
the product was purified by flash column 
 chromatography ( hexane  :  etoac 1  :  1 ) and the solvent evaporated to give 39  g ( a 
 yield of 78% ) as a colorless oil .
h 
 nmr ( 400  mhz , cdcl3 ) :  ( ppm ) 3.663.71 ( m , 12h , och2 ch2 ) , 3.40 ( t , 4h , j = 5.1  hz , ch2n3 ) .  
transformation to monoamine - terminated 
 teg ( 4 ) : 1,11-diazido-3,6,9-trioxaundecane 
 ( 3 ) ( 33.0  g , 135  mmol ) and 250  ml of 
 0.7 m h3po4 were placed in a 1  l round bottom flask and 
 cooled to 0c 
 using an ice bath while stirring .
a 
 solution of triphenylphosphine ( pph3 ) ( 29.0  g , 110  mmol ) in ether 
 ( 250  ml ) was slowly added via cannula and the temperature of the reaction was 
 maintained below 5c .
once addition was complete , the reaction 
 mixture was warmed to room temperature and stirred under nitrogen for an 
 additional 16 hours .
the biphasic 
 solution was separated and the aqueous layer was washed with ether ( 3  100 ml ) .
potassium hydroxide ( koh ) ( 30  g ) was slowly added to the aqueous layer and cooled to 0c 
 overnight .
the solution was filtered and the filtrate was basified with an additional 40  g of koh and cooled to room 
 temperature .
the reaction mixture was extracted with chcl3 ( 4  75 ml ) and the combined organic extracts were dried over mgso4 , filtered , and the solvent was evaporated to give 
 18.5 g of an oil with a slight yellow color . the monoamine transformation reaction has a yield of 63% .
h nmr ( 400  mhz , cdcl3 ) :  ( ppm ) 3.57 ( m , 10h , och2ch2 ) , 3.40 ( t , 2h , j = 5.2 hz , ch2ch2nh2 ) , 3.29 ( t , 2h , j = 5.0 hz , ch2n3 ) , 2.75 ( t , 2h , j = 5.2  hz , ch2ch2nh2 ) . coupling of amino - terminated teg to thioctic acid : ta - teg - n3 , 
 compound 5 : thioctic acid ( 11.1  g , 53.8  mmol ) , 
 1-amino-11-azido-3,6,9-trioxaundecane ( 4 ) 
 ( 11.2  g , 51.3  mmol ) , 4-dimethylaminopyridine ( 1.26  g , 10.3  mmol ) , and ch2cl2 ( 200  ml ) were placed in a 500  ml round bottom flask , cooled to 0c and 
 stirred under n2 atmosphere .
n , n-dicyclohexylcarbodiimide ( dcc ) 
 ( 11.1  g , 53.8  mmol ) was slowly added and reaction mixture was stirred at 0c for 
 2 hours , then warmed to room temperature and stirred for an additional 16 
 hours .
the reaction mixture was filtered 
 over a plug of celite and rinsed with etoac . 

the filtrate was evaporated and the crude material was purified by flash 
 column chromatography ( chcl3  :  meoh 97  :  3 as the eluent ) to give 11.7  g 
 ( yield of 56% ) of a yellow oil .
h 
 nmr ( 400  mhz , cdcl3 ) :  ( ppm ) 6.15 ( m , 1h , co  nh ) , 3.503.62 ( m , 11h , och2ch2 and ch ) , 
 3.47 ( t , 2h , j = 5.0  hz , co  nh 
ch2ch2o ) , 3.36 ( t , 2h , j = 5.5  hz , co  nh 
ch2 ) , 3.31 ( t , 2h , j = 5.0  hz , ch2n3 ) , 
 2.983.13 ( m , 2h , ch2 ) , 2.322.42 ( m , 1h , ch ) , 2.10 ( t , 2h , j = 7.4  hz , ch2co 
nh ) , 
 1.771.87 ( m , 1h , ch ) , 1.501.68 ( m , 4h , ch2 ) , 1.301.45 ( m , 2h , ch2 ) .
transformation to amine - terminated ta - teg , 
 compound 6 : 
 compound 5 ( 11.0  g , 27  mmol ) 
 was dissolved in thf ( 150  ml ) in a 300  ml round bottom flask fitted with a 
 condenser and nitrogen inlet .
pph3 ( 15.0  g , 81  mmol ) was added and the reaction was heated to reflux for 20 
 hours .
the reaction mixture was cooled 
 to room temperature and diluted with 15  ml of water and stirred for an 
 additional 20 hours .
the solvent was 
 evaporated and the crude product was purified by flash column 
 chromatography .
unreacted pph3 and triphenylphosphine oxide were removed with chcl3 and the eluent 
 was switched to chcl3  :  meoh  :  et3n ( 45  :  45  :  10 ) to collect the 
 desired product .
removal of the solvent 
 gave 9.0  g ( yield of 88% ) of gelatinous yellow oil .
h nmr ( 400  mhz , methanol - d4 ) :  ( ppm ) 3.413.59 ( m , 12h , och2ch2 ) , 3.26 ( t , 2h , j = 5.1  hz , co  nh 
ch2 ) , 
 3.22 ( m , 1h , ch ) , 2.973.12 ( m , 2h , ch2 ) , 2.71 ( t , 2h , j = 5.3  hz , ch2ch2nh2 ) , 2.322.42 ( m , 1h , ch ) , 2.12 ( t , 2h , j = 7.4  hz , ch2co 
nh ) , 
 1.751.85 ( m , 1h , ch ) , 1.471.69 ( m , 4h , ch2 ) , 1.291.44 ( m , 2h , ch2 ) .
biotinyl - n - hydroxysuccinimide 
 was first prepared following the synthetic procedure previously reported [ 1416 ] .
compound 6 ( 2.3  g , 6.0  mmol ) , biotinyl - n - hydroxysuccinimide 
 ( 2.05  g , 6.0  mmol ) were dissolved in dmf ( 50  ml ) and stirred at room 
 temperature under n2 atmosphere . 

et3n ( 3.0  g , 30  mmol ) was added dropwise via syringe and the 
 reaction was stirred for 16 hours .
the 
 solvent was evaporated under reduced pressure and the yellow residue was 
 purified by flash column chromatography ( using chcl3  :  meoh 95  :  5 as 
 the eluent ) .
the solvent was evaporated 
 to give 3.1  g ( a yield of 85% ) of viscous yellow oil .
h nmr ( 400  mhz , cdcl3 ) :  ( ppm ) 6.79 ( m , 1h , co  nh ) , 6.59 ( m , 1h , co 
ch ) , 3.52 ( m , 9h , och2ch2 and ch ) , 
 3.45 ( m , 4h , co  nh
 ch2 ) , 2.963.10 ( m , 3h , ch2 and ch  s ) , 2.762.83 ( m , 1h , ch 
s ) , 2.65 ( d , 1h , j = 12.8  hz , ch 
s ) , 2.302.40 ( m , 1h , ch ) , 2.11 ( m , 4h , nh  co
 ch2 ) , 1.751.85 ( m , 1h , ch ) , 1.451.68 ( m , 8h , ch2 ) , 1.261.42 ( m , 4h , ch2 ) .
compound 7 ( 2.0  g , 3.3  mmol ) was dispersed in a mixture of ethanol ( 40  ml ) and water ( 20  ml ) 
 while stirring in a 125  ml flask .
nabh4 ( 700  mg , 18.5  mmol ) was slowly added and the solution was stirred at room 
 temperature for 4 hours .
the reaction 
 mixture was diluted with water ( 200  ml ) and extracted with chcl3 ( 4  50 ml ) , dried over mgso4 , filtered , and the solvent was evaporated .
the 
 crude product was purified by flash column chromatography ( chcl3  :  meoh 
 95  :  5 as the eluent ) to give 1.53  g ( a yield of 77% ) of colorless viscous 
 oil .
nh ) , 6.69 ( m , 1h , co  nh ) , 
 6.63 ( s , 1h , biotin co 
ch ) , 3.50 ( m , 8h , och2ch2 ) , 3.42 ( m , 4h , co  nh 
s ) , 2.702.84 ( m , 2h , ch  s and ch  sh ) , 2.462.66 ( m , 3h , ch  s and ch2sh ) , 2.07 ( m , 4h , nh 
ch2 ) , 1.711.83 ( m , 1h , ch ) , 1.351.67 ( m , 13h , ch2 ) , 1.28 ( t , 1h , j = 8.0  hz , ch2sh ) , 1.23 ( d , 1h , j = 7.6  hz , ch 
core - shell qd 's used 
 were synthesized using high - temperature reaction of organometallic precursors 
 in a mixture of top / topo and alkylamine , as described in the literature [ 1720 ] .
cap exchange of the top / topo - capped qd 's with 
 the newly synthesized ligands to achieve water solubility ( a mixture of 1 and 8 , and a mixture of 2 and 8)   was carried out following 
 procedures described previously [ 5 , 12 , 21 ] . for cap exchange with a mixture of 2 and 8 , 50300 mg of top
/ topo - capped qd 's were precipitated using etoh and the supernatant was 
 discarded . to the precipitate 0.5  ml in 
 total of pure or mixed ligands and 0.5  ml of etoh
the mixture was then heated to 60   80c while 
 stirring for a period of 6 to 12 hours . 

once homogenized , the sample was then precipitated out with mixtures of 
 hexane , etoh , and chcl3 ( approximate ratio is 11  :  10  :  1 , this ratio may 
 vary from batch to batch ) ; the precipitate was dispersed in water .
the homogenized solution was further purified 
 using 3  4 cycles of concentration / dilution with an ultrafree centrifugal 
 filtration device ( millipore , mw cutoff 50,000 da ) to remove excess ligands and other materials from the solution .
cap exchange with a mixture of 1 and 8 was done similarly though some modifications are required for purification steps .  
dmf ( 5 ml ) was added to the reaction mixture after the solution was homogenized .
the precipitate was dispersed in water and the homogenized solution was further purified as described above .
briefly , samples were mixed with 30% glycerol , loaded into 1.5% agarose 
 gels buffered with 1x tris borate edta ( tbe ) in tbe running buffer ( ph 8.3 ) 
 and run at 10  volts / cm for 1 hour at ambient temperature . 

the gel - shift bands were visualized using the qd photoluminescence 
 collected on a kodak 440 digital image station ( rochester , ny , usa ) equipped 
 with a long - pass cutoff filter ; samples were illuminated with uv light ( 365  nm excitation ) .
the binding capacity of neutravidin - covered 96-well microtiter 
 flat - bottom plates from pierce biotechnology is 60 picomoles of biotin per 
 well .
100  l aliquots of qd samples ( with the 
 desired cap mixture at 7 pm concentration ) were added to the wells and let 
 incubate overnight at room temperature . 

the fluorescence intensity was measured using a tecan safire dual 
 monochromator multifunction microtiter plate reader ( tecan , research triangle 
 park , nc , usa ) , then plates were 
 washed 3 times with 10 mm na tetraborate buffer ( ph 9 ) supplemented with 0.02% 
 tween and the fluorescence signal was measured again ; 300  nm excitation 
 line was used for all the samples .
the chemical structures and synthetic schemes of a few representative ligands 
 ( namely dhla , dhla  peg600 , and dhla  teg  biotin ) and 
 reaction steps involved are summarized in figure 1 .
teg was first transformed into 
 diazide - terminated - teg ( 3 ) using a two - step reaction with methanesulfonyl chloride and sodium azide .
monosubstitution of one azide into an amine group was carried out in biphasic acidic solution to improve the efficiency of 
 selective formation of the monsubstituted product ( 4 ) .
thioctic acid ( ta ) and n3teg  nh2 ( 4 ) were coupled with n , n-dicyclohexylcarbodiimide ( dcc ) to provide azide - terminated compound 5 . 

the terminal azido group of 5 was reduced with pph3 and h2o to obtain an 
 amine - terminated ta - teg ligand ( compound 6 ) .
biotin - functionalized compound 7 was synthesized by coupling between 6 and biotin n - hydroxysuccinimide ester . 

finally , the terminal 1,2-dithiolane group in compound 7 was reduced with nabh4 to obtain 
 dhla terminal group as a bidentate thiol anchoring unit .
we verified quality of the new compounds by collecting h - nmr spectra 
 throughout the various steps employed . 

the h - nmr spectra show that teg - modified compounds have 
 large peaks around 3.6 ppm , which are ascribed to ch2 groups of teg 
 chains . after coupling between 6 and biotin n - hydroxysuccinimide ester , new distinct multiplet peaks ( 2.6  2.9 , 4.22 , and 4.41 ppm ) are measured in the nmr spectra , which were ascribed to 
 biotin - ring protons .
in addition , following reduction of the 1,2-dithiolane group with nabh4 , new 
 doublet and triplet peaks appeared at 1.23 and 1.28 ppm , respectively .
these two peaks were assigned as thiol 
 protons of dhla unit ( open dithiol ) .
the observed changes in each collected 
 spectrum indicate that each reaction step produced the desired compound .
peg600 or dhla  peg1000 ligands 
 can be dispersed in buffers with ph ranging between 5  12 . however , cap exchange of top / topo with dhla  teg 
biotin alone did not produce water - soluble qd 's , a property attributable to the rather short teg segment combined with the hydrophobic nature of the biotin group . to achieve water solubility using the new dhla 
biotin ligands 
 ( compound 8) , top / topo - capped qd 's were instead dispersed ( during cap exchange ) with mixtures of ligand 8 and either dhla ( 1 ) or dhla  peg600 ( 2 ) . rather low to modest
dhla  teg  biotin ratios ( 525% ) were used in the mixtures for cap exchange .
in addition to promoting easy transfer into buffer solutions , this approach ensures that only controlled and low density of 
 biotin groups are available on each qd .
this should also allow control over the number of biological receptors 
 coupled to a single qd and potentially biological activity of the qd - bioconjugates . in this particular 
 study , we used a mixture of compound 1 and compound 8 ( at 7  :  1 molar ratio ) , 
 and a mixture of compound 2 and compound 8 ( at 7:1 molar ratio ) 
 for the cap exchange . in the first characterization experiment of the cap exchange , we monitored changes in the electrophoretic 
 mobility of qd 's ( run on a 1.5% agarose gel ) as a function of the capping 
 mixture used .
the gel image in figure 2 
 shows a side - by - side comparison of the mobility shift of 510  nm emitting qd cap exchanged 
 with either a mixture of dhla : dhla  teg  biotin ( 7 : 1 ) ( lane 
 c ) or a mixture of dhla 
peg600 : dhla  teg  biotin ( 7  :  1 ) 
 ( lane d ) , together with control samples made of either qd 's capped with dhla 
 ( lanes a and e ) or qd 's capped with dhla 
the data clearly show that on the one   hand 
 qd 's capped with only dhla experienced the highest mobility shift ( towards the 
 positive electrode ) in this series ; this result is attributed to the presence 
 of carboxyl groups on dhla molecules , which can be deprotonated and negatively 
 charged in basic buffer solutions .
peg600-capped qd 's 
 showed no mobility shift under applied voltage ( materials did not migrate from 
 the loading well ) , indicating that these qd 's are essentially neutral ; water solubility 
 for this sample is mainly promoted by hydrophilicity of the long peg 
 chains . in comparison , the gel shift of 
 the qd with dhla : dhla  teg  biotin ( 7 : 1 ) was 
 slightly lower than that of the dhla - capped qd 's , whereas qd 's capped with dhla 
peg600 : dhla  teg  biotin ( 7  :  1 ) 
 mixture showed a very small shift compared with dhla  peg600-capped 
 qd 's . for qd 's capped with dhla : dhla  teg  biotin ( 7 : 1 ) mixture , 
 partial occupation of the nanocrystal surface by dhla 
teg  biotin ( compared 
 with dhla - capped qd 's ) has led to reduced number of charges per qd and thus 
 small reduction in the gel mobility shift ( lane c ) .
the nonzero mobility shift ( even though 
 extremely small ) measured for qd 's capped with a mixture of dhla 
peg600  :  dhla  teg  biotin may be due 
 to a slight / partial charging of biotin or amide groups on the chain under 
 applied voltage .
overall , the present 
 gel electrophoresis data confirm that cap exchange of top / topo - capped qd 's with 
 a ligand mixture produced nanocrystals that have both types of capping ligands ; 
 furthermore , the overall relative proportions of each ligand used during cap 
 exchange is preserved in the final qd samples .
absorption 
 and fluorescence spectra were measured for both the native top / topo - capped qd 's 
 in toluene and the new hydrophilic qd 's capped with dhla 
absorption spectra measured before and after the cap exchange were 
 essentially unchanged , though a few nm red shift of the lowest absorption 
 maximum of the hydrophilic qd 's was occasionally measured compared with that of 
 qd 's capped with top / topo ligands .
these occasional small changes in the optical properties of qd 's 
 following transfer into aqueous solutions are commonly observed [ 1 , 12 , 23 ] .
the fluorescence quantum yields of the 
 water - dispersed qd 's change from batch to batch . 

the quantum yields in aqueous solutions ( after cap exchange and 
 transfer ) are 50% of the original values measured for top / topo - capped qd 's in 
 organic solutions .
biotin ligands 
 were also stable and aggregate - free over extended periods of time 
 ( months ) .
figure 3 shows solutions of 
 qd 's capped with a mixture of compound 2 and compound 8 ( 7  :  1 ) in h2o of which ph ranged from 5.5 to 9
. the 
 new water - soluble qd 's were stable and well dispersed in the wide ph range 
 including acidic conditions .
this 
 feature is quite different from what we have observed for solutions of qd 's 
 capped with either dhla or mercaptoundecanoic acid ( mua ) .
the dhla- or mua - capped qd solutions were 
 well dispersed in aqueous basic media but showed aggregation and eventual 
 precipitation at ph lower than 7 ( data not shown ) , indicating that 
 deprotonation of carboxyl groups is a crucial factor to solubilize those qd 's in 
 aqueous solutions .
in contrast , appending a short peg chain at the end of dhla 
 molecules extended the stability of qd capped with either 100% dhla  peg600 or a 
 mixture of dhla 
once 
 cap exchange with biotin - terminated dhla  teg and transfer 
 into aqueous environment was successfully realized , targeted biological assays 
 were carried out .
teg ( 8) were investigated for their ability 
 to interact with neutravidin - functionalized substrates .
neutravidin , which is a deglycosylated form 
 of avidin , was chosen due to its low nonspecific interactions ( compared with 
 avidin or streptavidin ) while maintaining the strong binding affinity with 
 biotin . following incubation of the neutravidin - functionalized 
 substrates with the qd samples ,
substrates were rinsed 3 times with borate 
 buffer with tween and the fluorescence signals were collected .
figure 4 shows the fluorescence intensities 
 measured for 4 different qd solutions incubated with neutravidin - coated plates : 
 one capped with dhla ( 1 ) , one capped 
 with dhla 
peg600 ( 2 ) , one capped with a mixture of 1 and dhla  teg  biotin ( 8) ( at 4  :  1 ratio ) , and one capped with 
 a mixture of 2 and 8 ( at 4  :  1 ratio ) .
the data clearly show that only substrates 
 incubated with qd 's that were capped with a mixture of biotin - terminated dhla  teg produced a 
 large signal - to - background fluorescence signal . 
 in comparison
, the substrates incubated with dhla- or dhla  peg600-capped qd 's 
 produced only background contribution to the signal .
this result indicates that efficient and 
 specific interactions between the biotin groups present on the qd surface and 
 neutravidin on the substrate drive the surface capture of the 
 nanocrystals .
biotin alone 
 were , however , not dispersible in buffer solutions , a result we attribute to 
 the poor water compatibility of biotin by itself and the short teg .
we are exploring the use of longer chain peg 
 segments for insertion between the dithiol group and biotin .
this should make dispersion in buffer 
 solution achievable even at high biotin fractions and promote strong 
 interactions with neutravidin .
we have demonstrated simple and 
 efficient synthetic procedures to prepare new biotin - functionalized ligands 
 based on the dhla motif and employing short teg segment .
the present synthetic route provided high 
 quality and stable compounds , which were further employed to make 
 biotin - functionalized luminescent qd 's , using easy - to - implement cap exchange 
 procedure .
peg600 
 effectively cap exchanged with the native top / topo and provided qd 's that are 
 water - soluble over extended periods of time and biologically active .
peg600 ( neutral ) 
 and dhla  teg  biotin showed 
 specific interactions with neutravidin in surface binding assays .
the present synthetic methodologies of 
 hydrophilic surface ligands and cap - exchange reactions promise access to a 
 variety of biological entities .
further 
 studies of these surface - functionalized qd 's for coupling with a variety of 
 bioreceptors and biological assays are in progress .
 we report the design and synthesis of a tetraethylene glycol- ( teg- ) based bidentate ligand functionalized with dihydrolipoic acid ( dhla ) and biotin ( dhla  teg  biotin ) to promote biocompatibility of luminescent quantum dots ( qd 's ) .   
 this new ligand readily binds to cdse  zns core - shell qds via surface ligand exchange . 
 qds capped with a mixture of dhla and dhla  teg  biotin or polyethylene glycol- ( peg- ) ( molecular weight average 600 ) modified dhla ( dhla  peg600 ) and dhla  teg  biotin are easily dispersed in aqueous buffer solutions . in particular , homogeneous buffer solutions of qds capped with a mixture of dhla  
 peg600 and dhla  teg  biotin that are stable over broad ph range have been prepared . 
 qds coated with mixtures of dhla / dhla  teg  biotin and with dhla  
 peg600/dhla  teg  
 biotin were tested in surface binding assays and the results indicate that biotin groups on the qd surface interact specifically with neutravidin - functionalized microtiter well plates . 
1. INTRODUCTION
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION
4. CONCLUSION