there is a critical 
 need for clinical probes that can simultaneously 
 assist in early detection , diagnosis , imaging , and therapeutic treatment 
 of disease . silicon nps have been shown to have minimal toxicity 
 and are easily passivated and inexpensive to produce . unlike the ii  vi or iii v quantum dot ( qd ) materials , 
 si is a bioinert semiconductor that is electrochemically stable . si 
 nanoparticles have visible luminescence properties caused by a radiative 
 recombination of an exciton ( electrons and holes ) in si . free - standing luminescent si nanoparticles 
 have been prepared using a variety of physical and chemical techniques , 
 including ultrasonic dispersion of electrochemical etching , laser - driven pyrolysis of silane , ion implantation , synthesis in supercritical fluids , metal organic 
 chemical vapor deposition ( mocvd ) , pulverizing 
 and sonicating p - si , and solution - based precursor 
 reduction . significant advances in the preparation of si nanoparticles have 
 been realized in the past two decades . however , two important issues that remain are the reliance of many 
 of the existing methodologies on specialized reagents or reactors 
 and the need for hazardous hf to afford efficient control over particle 
 size . to date , only a few studies have 
 reported the colloidal synthesis of water - dispersible si nanoparticles , and most of the studies report si nanoparticles used solely for 
 fluorescence imaging . manganese - doped group iv 
 semiconductors have attracted attention 
 because of their relevance in current device technology . mn ions have been proven to be a useful magnetic resonance 
 ( mri ) contrast agent for functional imaging as a noninvasive diagnostic for detection of disease . the addition 
 of paramagnetic mn to si nanoparticles would allow combinations 
 of optical detection with magnetic resonance imaging techniques or magnetic separation . mn ions are similar to other paramagnetic ions since 
 they are capable of shortening the t1 of 
 water protons . in addition , mn can also have a t2 effect , influencing the signal intensity by 
 shortening the transverse relaxation time to produce negative contrast . mn has potential as useful contrast agents and can 
 be a valuable tool in obtaining more information to get detailed physiological 
 and biochemical information from mri . recently , we have reported that for biological applications dual - modality 
 imaging methods have shown potential for significantly increased diagnostic 
 accuracy compared to stand - alone imaging . this has led to increasing interest in multifunctional materials 
 that can be detected by both modalities . incorporating dopants 
 into nanostructures is difficult due to their 
 small size and large surface - to - volume ratio which is an effective 
 barrier to impurity doping . also , the 
 doping of either n- or p - type impurities 
 can cause the quenching of the photoluminescence ( pl ) because of the 
 efficient auger interaction between photoexcited electron typically , for group iv semiconductors , homogeneous doping of mn is difficult , largely due to the extremely low solubility 
 of mn in crystalline group iv semiconductors near room temperature . water - soluble mn - doped silicon ( simn ) and fe - doped silicon 
 ( sife ) nanoparticles have been prepared and characterized 
 for biological applications . in the case of simn nanoparticles , a major challenge to synthesis has been the low molar 
 ratio of mn in the nanoparticles ( 0.05% ) . in an effort to increase the concentration of mn in the 
 si nanoparticle , we have employed manganese(ii ) acetylacetonate 
 as the source of mn . in this paper , we focus on the synthesis and 
 characterization of the nps and epr characterization of the mn site 
 in water - soluble , paramagnetic simn nanoparticles . the 
 concentration of mn in simn nps afforded by using the organic 
 manganese salts is improved 10-fold over our previous reports . electron paramagnetic resonance ( epr ) illuminates 
 the location of mn in the simn nps . these simn nps were passivated with dextran in order to reduce their overall 
 surface charge and improve solubility in anticipation of in 
 vivo bioapplications . these dextran - coated simn nps were shown to have potential 
 as an effective t2 contrast agent . sodium hydride powder ( nah , 95% ) , silicon 
 powder ( si , 99% ) , anhydrous manganese(ii ) acetylacetonate powder 
 ( mn(acac)2 , 99.99% ) , and allylamine ( c3h7n , 98% ) were purchased from sigma - aldrich and were used without 
 further treatment . ammonium bromide ( nh4br , 99.99% ) 
 and n , n - dimethylformamide 
 ( dmf , 99.8% ) were purchased from sigma - aldrich and dried before use . water was purified using a nanopure analytical uv water system ( 18.0 
 m cm , barnstead ) . all manipulations were carried out under 
 dry n2 or ar gas , in a glovebox , a tube furnace , or schlenk 
 line , using standard anaerobic and anhydrous techniques . different 
 mn - doped na4si4 precursors were 
 prepared according to a modified previously published procedure from 
 na4si4 . mixtures 
 of nah , si , and mn powders with the molar ratios ( mnah : msi : mmn = 1.9:(1  x):x , and x = 1% , 3% , 5% , and 7% ) and nah , si , and mn(acac)2 powders with the molar ratios ( mnah : msi : mmn = 
 1.9:(1  x):x , and x = 1% , 3% , and 5% ) were mixed in a spex 8000 m mill . a tungsten 
 carbide vial , two tungsten carbide balls ( diameter of 1 cm ) , 
 and one tungsten carbide ball ( diameter of 5 mm ) were used 
 to mill the mixtures . the milling was performed for 1 h ( using 15 
 min on , 15 min off intervals for a total of 30 min ) to obtain a homogeneous 
 powder , which was transferred back into the glovebox . the mixtures 
 ( nah , si , and mn powder and nah , si , and mn(acac)2 ) were 
 placed into separate 1 ml alumina crucible boats using 
 a second 1.5 ml alumina boat as a cover . the quartz tube was 
 removed from the glovebox and placed in a horizontal tube furnace 
 and connected to ar flowing at 30 ml / min . the quartz tube was rapidly 
 heated ( 180 c / h ) to 420 c and maintained for 48 h followed 
 heating at 180 c / h to 500 c and dwelled for an additional 
 24 h. after cooling to room temperature , the tube was transferred 
 back into the glovebox where the black solid was extracted and stored . caution : nah and mn - doped na4si4 powder 
 are highly reactive to moisture and must be handled under an inert 
 atmosphere ! a two - necked 500 ml schlenk flask was placed 
 into the glovebox , and 
 na4si4xmnx ( 0.25 g , 4.9 mmol ) and nh4br ( 0.50 g , 
 5.1 mmol ) were added and taken to a schlenk line . the mixture was allowed to 
 reflux overnight under an inert atmosphere . after the reaction mixture was cooled to room temperature , 2 ml of allylamine was added , and 
 the reaction mixture was allowed to reflux vigorously overnight while 
 stirring . this mixture was allowed to cool to room temperature , and 
 the mixture was centrifuged . the supernatant , a yellow solution , was 
 decanted into a round - bottom flask . the solvent was exchanged with 
 nanopure water using a rotavap followed by dialysis against nanopure 
 water to purify the product of any unreacted and excess salts present . the resulting orange viscous product was filtered through a syringe 
 filter to remove any solid impurities and was utilized for further 
 characterization and studies . sodium tetraborate decahydrate was 
 used to prepare a borate buffer for the ph stability study . hydrochloric 
 acid and sodium hydroxide were added to adjust the ph from 3 to 10 
 for a total of eight samples . thirteen microliters was added to 3 
 ml of each acid and base from ph 3 to ph 10 . the particles were allowed 
 to sit for 2 , 4 , 6 , 8 , 24 , 48 , and 96 h , and at each time point the 
 absorbance and emission were recorded . the eight samples were placed 
 in an incubator for 2 , 4 , and 48 h at 37 c . following the incubation 
 the samples were placed in a sandbath at approximately 90 c 
 for 4 h. the x - ray powder diffraction data 
 were collected with a bruker d8 advance diffractometer using cu k 
 radiation ( 40 kev , 40 ma ) . the samples were ground into powders and 
 measured using a commercial sample holder supplied by bruker for air - sensitive 
 materials or a quartz plate for the precipitates from the reactions . data were collected on a rotating sample ( 15 rpm ) from 20 to 
 80 ( 2 ) , with a step size of 0.019  44 and 
 a step time of 20 s for the air sensitive and 2.4 s for the non - air - sensitive 
 samples , respectively . mdi jade was used for phase identification ; full pattern fitting for 
 the present and identified phases was performed using the fullprof 
 suite . elemental analysis was performed 
 by icp quadrupole mass spectrometers ( icp - ms , agilent technologies 
 7500ce ) . transmission electron microscopy ( tem ) was performed on a 
 philips cm-12 , operating at 80 kv . tem samples were prepared by dipping 
 holey - carbon - coated , 400-mesh electron microscope grids into the aqueous 
 solution of the product and drying them in air . high - resolution transmission 
 electron microscopy ( hrtem ) was carried out using a jeol 2500se schottky 
 emitter microscope operated at 200 kv with a gatan multiscan camera 
 associated with it . infrared spectra were obtained by 
 dropping the liquid onto the plate of a bruker optic gmbh alpha atr - ftir . photoluminescence ( pl ) spectra were measured on a fluoromax-3p fluorometer , 
 where the mn - doped si nanoparticles were dispersed in water . longitudinal ( t1 ) and transverse ( t2 ) relaxation times were measured at 60 mhz 
 ( 1.4 t ) and 37 c on a bruker minispec mq60 ( bruker , billerica , 
 ma ) . the solutions of mn - doped si nanoparticles were prepared by dissolving 
 appropriate amount of samples in ph 7.0 deionized water . t1 values were measured using an inversion recovery sequence 
 with 1015 data points , and t2 values 
 were measured using a carr  purcell  meiboom gill 
 ( cpmg ) sequence with  = 1 ms and 200 data points . each solution 
 was incubated at 37 c for 10 min before measurement and measurements 
 performed at 37 c . the longitudinal ( r1 ) and transverse ( r2 ) relaxivity 
 were determined as the slope of the line for plots of 1/t1 or 1/t2 , respectively , against 
 increasing manganese concentration with a correlation coefficient 
 greater than 0.99 . magnetic resonance imaging ( mri ) was performed 
 on a bruker vertical 
 bore 9.4 t ( 400 mhz ) microimaging system , 21 c , bruker , billerica , 
 ma ) . the magnet was equipped with the standard gradient set ( 95 mt 
 m maximum gradient ) and 25 mm internal diameter 
 ( i.d . ) parameters for the images were tr = 500 ms and 
 te = 5 ms . for all images a flash-2d sequence was used with a field 
 view ( fov ) of 4.40  4.45 cm , slice thickness 1.0 
 mm , and a 128  128 matrix . the electron paramagnetic resonance 
 ( epr ) spectra for the samples 
 were collected on a bruker ecs106 x - band spectrometer , equipped with 
 an oxford instruments liquid helium cryostat . typical experimental 
 conditions were frequency 9.68 ghz , temperature 10 k , modulation amplitude 
 10 g , microwave power 63 w , conversion time 40.96 ms , time 
 constant 40.96 ms , resolution 2048 pts , and average of 10 scans . varying concentrations of mn - doped si ( simn ) nps were made by the reaction of the mn - doped precursor na4si4 and nh4br in dmf and capped with 
 poly(allylamine ) . adapting a previously published procedure for m - doped 
 na4si4 , na4si4 m ( m = mn , 
 p , fe ) , mixtures of 
 nah , si , and mn metal powders or mn(acac)2 powders with 
 different molar ratios were explored to determine if the mn powder 
 or a mn complex would provide an increase of the mn doping concentration 
 thus leading to differing physical properties . mn metal powder had been employed previously , and in this study the mn(acac)2 complex is investigated 
 as a potentially better starting material for mn incorporation . three different starting concentrations of mn(acac)2 complex 
 ( 1 , 3 , and 5% , noted as a , b , and c ) were employed to produce mn - doped sodium silicide ( na4si4mn).figure 1b shows the reaction equation using the metal complex mn(acac)2 . after reaching the final temperature of 500 c , the 
 main phase formed is na4si4mn . ( a ) schematic representation 
 of mn - doped zintl phase na4si4mn with mn metal 
 powder . powder x - ray diffraction 
 was used to characterize na4si4mn precursors 
 ( supporting information ) . table 1 provides 
 the lattice parameters for the main phase , na4si4mn . as the amount of mn(acac)2 is increased , thermally stable 
 binary manganese silicide phases are formed , and for the highest mn 
 content , a clathrate phase is observed in addition to na4si4mn . the lattice parameters obtained for the three different 
 preparations of mn doping are consistently larger than that for pristine 
 na4si4 ( monoclinic , c12/c1 ( no . 15 ) , a = 12.1536(5 ) 
  , b = 6.5452(5 )  , c = 
 11.1323(6 )  ,  = 118.9(1) ) . the volume increases from 780.66  for preparation 
 a to 784.18  for preparation c ; thus , we concluded 
 that mn is incorporated into the structure of na4si4 . simn nps are produced according to the 
 reaction scheme 
 in figure 1c , and the 
 resulting product is composed of two phases : a solid and liquid phase . 
 the liquid phase is further purified while the precipitate is washed 
 with methanol and characterized by x - ray diffraction . although the 
 precipitate is not used further , identifying the products in the precipitate 
 provides insight into mn incorporation into the solution 
 containing poly(allylamine)-coated simn nps . the precipitate 
 shows the presence of crystalline si along with different manganese 
 silicides and other phases ( supporting information ) . the identified phases for the precipitate in all reactions are 
 si with increasing amounts of mn4si7 . si phase diagram 
 that manganese silicides are formed with an excess of either silicon 
 or mnsi.table 2 provides the lattice parameters for the 
 precipitated si obtained in these reactions and shows that while the 
 lattice parameter increases for preparations a and b ( 1 and 3% mn ) , 
 it decreases for preparation c , the 5% initial composition of mn . this result suggests there is a threshold of how much mn can be incorporated into si by this method . in addition 
 to the lattice parameters , table 2 provides the composition of the mn in 
 the solution for the simn nps . as the concentration of 
 mn(acac)2 is increased from 1 to 5% in the na4si4 precursor , the highest concentration obtained for 
 mn in the simn nps in solution was found to be 0.4  
 0.3% . this is a 4 times greater mn content compared to the nanoparticle 
 product from mn powder which yielded a maximum of 0.1 mol % . the mn complex allows for an increase in the 
 mn concentration in the nps , which did not occur when using mn powder . mn(acac)2 has the advantage of being more precisely weighed 
 because of its molecular weight and better distributed in the si powder 
 via the ball milling than mn metal powder which typically has a thin 
 oxide coating . since the sample is heated under flowing argon , the 
 organic acetylacetonate is easily decomposed and removed from the 
 sample . the lattice parameters obtained for the solid precipitate 
 are consistent with the chemical analysis of the simn np 
 solution , suggesting that the highest mn content is for 
 the materials preparation b with 3% mn(acac)2 . the morphology and size of simn nps from mn(acac)2 ( preparation b ) were obtained by both low- and high - resolution 
 tem and eds ( figure 2 ) . in figure 2a , the 
 inset indicates a histogram from the high resolution data with the 
 average diameter of 4.2  1.0 nm , consistent with previously 
 reported average sizes that are obtained from this reaction . the lattice fringes of 1.87  were consistent with the ( 220 ) 
 silicon planes ( figure 2b ) . lattice fringes measured from several hrtem images verified that 
 the nps are diamond structured si ( supporting information , figure 4 ) . selected area electron diffraction ( saed ) is also consistent 
 with crystalline si ( figure 2c ) . energy dispersive x - ray spectroscopy ( eds ) however , mn was not detected presumably 
 due to the low concentrations within the nps . ( a ) tem of simn nanoparticles preparation b with a histogram 
 inset indicating an average size of 4.2  1.0 nm . ( b ) high - resolution 
 tem showing the ( 220 ) lattice plane for si . figure 3 shows the 
 ft - ir attenuated total reflectance ( atr ) absorption spectrum of poly(allylamine)-coated 
 simn nps produced from the mn(acac)2 reaction . an n  h stretch is observed at 3393 cm with 
 ch2 stretches at 3018 and 2929 cm . an nh2 deformation can be seen at 1660 , 1640 , and 1531 
 cm . n stretch is at 998 cm , and the ch2 rocking is at 934 cm . this spectrum is similar to what is found in the literature for 
 poly(allylamine ) . a photoluminescence study was used to determine possible decomposition 
 based on a decrease in emission after treatment . the particles were 
 allowed to sit for periods of time at increasing ph , and the emission 
 was recorded . a quantum yield of 1115% was observed ( supporting information ) . in a second experiment , 
 the samples were heated at 37 c for periods of time and the 
 quantum yield was maintained at 1115% , suggesting high stability 
 of the nps . however , higher temperatures ( 90 c ) for 4 h resulted 
 in a significant decrease in quantum yield to about 5% . figure 4 shows the t1 and t2 weighted mri of phantoms of preparation b 
 simn nps and the optical emission of the same samples excited 
 at ex = 350 nm . the maximum emission is red - shifted 
 and reduced as the concentration of the sample increased , indicating 
 aggregation and quenching of the intensity . this result is similar 
 to that observed for iron - doped si , sife , nps . the intensities of the mri phantoms demonstrate 
 the brightening ( top row ) of t1 weighted 
 images and the darkening ( bottom row ) of t2 weighted images vs concentration . thus , simn nps are 
 both t1 and t2 relaxation agents and can be used in t1 or t2 weighted applications to deliver 
 drugs coordinated in the polymer or conjugated through the amines 
 for several different types of imaging . these data indicate that at 
 least some of the mn ions are near the surface of the 
 np in order to relax water protons via inner- and outer - sphere mechanisms 
 for t1 weighted contrast . simn nanoparticles 
 preparation b ( 0.4  0.3 mn : si 
 mol % ) , and concentrations are indicated for mn . ( a ) t1 and t2 weighted 
 mri of phantoms and ( b ) emission intensity measured at 350 nm . table 3 provides 
 the mn content in mol % ( mn : si ) and the measured relaxivities with 
 and without dextran coating in mm per mn . the preparation 
 label is the same as for table 2 ; however , since we want to correlate the results with the 
 amount of mn in the colloid , the order changes . both 
 mn concentration and the dextran coating vary the relaxivities r1 and r2 . the fits 
 to the relaxation data vs concentration that yield the values in table 3 are shown in the supporting information . previously reported samples of simn nps 
 equivalent to 
 preparation a , but prepared using mn metal powder to form na4si4mn , were dextran sulfate coated and had an r2/r1 of 3.49 ( with 
 an r1 relaxivity of 25.50  1.44 
 mm s and an r2 relaxivity of 89.01  3.26 mm s with respect to [ mn ] ( 37 c , 
 1.4 t ) ) , indicating their use as a positive contrast agent . preparations b and c of simn nps 
 prepared with mn(acac)2 were chosen to be coated with dextran 
 since the poly(allylamine)-coated particles showed promising relaxivities . an r1 relaxivity similar to the previously 
 published results , 27.1  2.8 mm s , was obtained but showed a larger r2 relaxivity of 1078.5  1.9 mm s. likewise , preparation c showed similar results with a large r2 of 822.4  77.9 mm s. the measured relaxivities are consistent 
 with x - band electron paramagnetic 
 resonance ( epr ) results . epr spectra of the nps from the three preparations 
 are shown in figure 5 . epr spectra of the three different preparations a  c of mn - doped si nps , simn . these are classic looking mn epr spectra with s = 5/2 and i = 5/2 . the epr spectra exhibit 
 six large peaks that are the allowed ( ms = 1 ) transitions , and the smaller 
 doublets between are forbidden transitions ( ms = 1 and mi = 1 ) . isotropic a and g tensors were used along 
 with the zero field splitting parameters d and e ( eq 1 ) . strain 
 in d and e was also incorporated . the data can not be fit with one component . to reproduce the long tails 
 on either side of the lines the epr parameters g , a , [ de ] , [ d strain e strain ] , and line 
 width were varied , albeit not simultaneously . the two - component fits 
 were significantly better than fitting only one component in every 
 case , and for preparation c , a third spin 1/2 component was required . , the two - component fits reproduce the epr 
 spectrum significantly better in the region from 350 to 360 mt . the 
 fits yielded a hyperfine coupling ( a ) value of 265 
 mhz for component type 1 for all three doping concentrations , which 
 is similar to previously published results on si - doped mn nanoparticles , while for type 2 , a 
 value of 238 mhz was obtained . in the case of mn doping there was also a clear s = 1/2 signal at g  2 in an initial data set taken with a 1 g field 
 modulation ( data not shown ) . the 10 g modulation used for acquiring 
 the data for fitting diminishes the spin 1/2 signal to the point where 
 it is not evident . thus , a third species of spin 1/2 was also included 
 in the fitting . these data are consistent with previous epr on similarly 
 prepared si : mn np also acquired with 10 g modulation . the 
 fitting of the x - band data is very slow , indicating shallow minima 
 in the least - squares fitting . the data 
 are shown in black , one - component fit is in green , and two - component 
 fit is in red . expanded view shows the feature that the two - component 
 fit reproduces that the one - component fit does not . the epr parameters of mn doped into 
 si take on a variety 
 of values depending on sample preparation . both charge and spin state may 
 be adjusted via doping , and each combination exhibits own hyperfine 
 interaction . s = 5/2 is the natural state on mn in si without doping , and s = 5/2 mn has been observed in si in both interstitial and substitutional 
 sites with a = 160 and 121 mhz , respectively . muller et al . indicate that rapid quenching 
 of transition metals in si always produces interstitial type impurities . note 
 that the values for a obtained in table 4 are about a factor of 2 larger 
 than the studies cited above where mn is doped into single 
 crystals but consistent with the previous report on simn nps . both interstitial and substitutional 
 sites have tetrahedral symmetry , 
 and thus d and e terms are not expected 
 unless there is deviation from that symmetry . incorporation of the 
 zero - field splitting ( zfs ) accounts for the crystal - field - induced 
 lowering from cubic ( or higher ) symmetry . this term can be expanded 
 ( to first order ) as a function of two empirical parameters , d ( the axial contribution to the zfs ) and e ( the rhombic contribution):1d and d strain 
 ( and thus e and e strain ) were necessary 
 for adequate fits and this is the first ( that we know of ) reported 
 values of d and d strain in mn - doped 
 si . we note that d and d strain 
 have been used in fitting epr spectra of mn : cdse quantum dots . previous studies of mn doped into si , which 
 were performed on single crystalline si , showed mn doping levels several 
 orders of magnitude lower than achieved here . thus , the mn impurities were in sites of tetrahedral symmetry . here , 
 the small size ( 4 nm diameter ) of the particles would argue 
 that a significant fraction of the mn atoms are not in a site of tetrahedral 
 symmetry due to being near the surface . moreover , the high mn concentration 
 indicates there are multiple mn per np ( at 1% there are on average 
 9.5 mn atoms per np ) . furthermore , the standard theory for 
 epr parameters on mn in si demonstrates that next - nearest neighbors 
 are important to the understanding of epr spectra . thus , the existence of nearby electron spins in the form 
 of another manganese and loss of tetrahedral symmetry for sites near 
 the edge provide sufficient justification for the incorporation of d and e and their respective strains into 
 the fits . an important result 
 from the fitting is that we obtain the relative 
 amounts of the two types of sites . we note that for plain nps there 
 is a mild correlation between the type 2 to type 1 ratio and the r1 relaxivity whereas there is a strong correlation 
 between the type 2 to type 1 ratio and the r2 relaxivity data ( table 4 ) but not measured mn content . what is unclear from 
 these data is the physical location of these two sites . the first is that a 
  surface  type is increasing with the type 2:type 1 
 ratio , placing the magnetic moment closer to the water . we note that 
 on a np of 4.2 nm a shell of 0.4 nm ( the si next - nearest - neighbor 
 distance ) occupies approximately 1/4 of the volume of the np . the high relaxivity 
 argues for the larger fraction of type 2 mn to be near 
 the surface . furthermore , the d values obtained for 
 type 2 are a factor of 2 larger than that of type 1 , which is what 
 one would expect if the mn atoms are near the surface due to lower 
 symmetry this 
 lack of correlation of relaxivity with measured mn content would be 
 consistent with the correlation with type 2/type 1 ratio if the different 
 preparations produced different distributions of mn within 
 the nps . differences in average size of the nps may also play a role , 
 leading to different distributions of mn . another possibility 
 is that the spin 1/2 signal observed in preparation c is responsible 
 for the large r2 . r1 and r2 for the dextran - coated 
 simn nps both have a negative 
 correlation with type 2:type 1 ratio but correlate with measured mn concentration . of note is the particularly large r2 . the large r2 combined 
 with the ability to be further conjugated with target specific molecules 
 makes them attractive for future bioapplications . a higher percentage of mn incorporation into si nanoparticles was 
 achieved by using mn(acac)2 to form the starting material , 
 na4si4mn . x - ray diffraction supports the claim 
 that mn is incorporated in the starting material , na4si4mn , due to the increased lattice parameters . incorporation 
 of mn into the simn nps is assumed since the remaining 
 precipitate consists of manganese containing phases . the simn nps also show 
 stability for extended periods of time throughout broad ph ranges . epr indicates a two - component system where the mn ions 
 occupy two different sites within the nps and that the sites are of 
 lower symmetry compared to mn doped into crystalline 
 bulk si . moreover , since r1 and r2 are large , at least one type of mn is probably close to the surface . characterization of the poly(allylamine)-coated 
 si nps showed promising relaxivities for bioapplications . a next logical 
 step is a surface modification with target specific molecules to probe 
 the application in biological systems .
water - soluble 
 poly(allylamine ) mn2 + -doped si ( simn ) nanoparticles 
 ( nps ) were prepared and show promise for 
 biologically related applications . 
 the nanoparticles show both strong 
 photoluminescence and good magnetic resonance contrast imaging . the 
 morphology and average diameter were obtained through transmission 
 electron microscopy ( tem ) and high - resolution transmission electron 
 microscopy ( hrtem ) ; spherical crystalline si nps with an average diameter 
 of 4.2  0.7 nm were observed . 
 the doping maximum obtained through 
 this process was an average concentration of 0.4  0.3% mn per 
 mole of si . 
 the water - soluble simn nps showed a strong 
 photoluminescence with a quantum yield up to 13% . 
 the simn nps had significant t1 contrast with 
 an r1 relaxivity of 11.1  1.5 mm1 s1 and r2 relaxivity of 32.7  4.7 mm1 s1 where the concentration is in mm of mn2 + . 
 dextran - coated poly(allylamine ) simn nps produced nps 
 with t1 and t2 contrast with a r1 relaxivity of 27.1 
  2.8 mm1 s1 and r2 relaxivity of 1078.5  1.9 mm1 s1 . 
 x - band electron paramagnetic resonance spectra 
 are fit with a two - site model demonstrating that there are two types 
 of mn2 + in these np s . 
 the fits yield hyperfine 
 splittings ( a ) of 265 and 238 mhz with significant 
 zero field splitting ( d and e terms ) . 
 this is consistent with mn in sites of symmetry lower than tetrahedral 
 due to the small size of the np s .