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recently , fluorescent - magnetic bifunctional nanomaterials which are composed of magnetic iron oxide nanoparticles and luminescent dye - doped silica matrix gained more and more attention [ 1 - 10 ] . on the one hand , superparamagnetic iron oxide nanoparticles including maghemite ( -fe2o3 ) and magnetite ( fe3o4 )
were widely investigated for in vivo and in vitro biomedical applications , such as magnetic resonance imaging ( mri ) , target drug delivery , and so on [ 11 - 14 ] . on the other hand ,
dye - doped silica nanoparticles were good candidate for bio - labeling and bio - imaging because they showed several advantages , including photostable , sensitive , water soluble , and easy surface modification [ 15 - 17 ] .
so these bi - functional nanoparticles could provide fluorescent and magnetic properties simultaneously which make them useful in highly efficient human stem cell labeling , magnetic carrier for photodynamic therapy , and other biomedical applications [ 1 - 3][5 - 8 ] .
up to now , several methods have been developed for preparing such fluorescent - magnetic bi - functional nanomaterials [ 2 - 5 ] .
have conjugated dye - doped silica with iron oxide nanoparticles by surface modification method . alternatively
, organic dye - incorporated silica shell - coated iron oxide nanoparticles can be prepared in a reverse micelle system .
these strategies could produce high quality fluorescent - magnetic nanoparticles , but they either needed expensive reagents or complicated synthetic steps .
have prepared inorganic dye - doped silica shell - coated iron oxide nanospheres by stber method which needed fewer organic solvents and the preparation procedure was convenient . but compared with inorganic dye , organic dye molecules seem to be better option for bio - labeling and bio - analysis because of their relatively high intrinsic quantum yield .
so , simple and economic method for preparing organic dye - impregnated silica shell - coated iron oxide nanoparticles is still needed to be developed .
recent studies indicated that hydrophobic silane was a good candidate to entrap organic dye into the silica matrix .
so we developed a new method for preparing organic dye - impregnated silica shell - coated iron oxide nanoparticles based on the hydrolysis of htmos and teos .
iron oxide nanoparticles were first coated with a dye - impregnated silica shell by the hydrolysis of htmos which produced a hydrophobic environment for entrapping organic dye molecules ( rhodamine 6 g was used as model dye ) .
subsequently , the particles were coated with a hydrophilic shell by the hydrolysis of teos , which enabled the resulting nanoparticles to be dispersed in aqueous solution .
herein , the synthesis procedure and the characterizations of the final multifunctional nanomaterial were summarized in detail .
iron oxide nanoparticles were prepared by adding ammonia to an aqueous solution of fe / fe at a 1:2 molar ratio .
. then the iron oxide nanoparticles were coated with rhodamine 6 g doped silica shell .
typically , 0.75 ml of s1 , 1.5 ml of h2o , 0.6 ml of ammonia , and 10 ml of isopropyl alcohol were mixed together under magnetic stirring .
subsequently , 5 ml of rhodamine 6 g solution in isopropyl alcohol and appropriate volume of htmos was added into the mixture . after stirring for 3.0 h
, 5 ml of isopropyl alcohol and 80 l of teos were added into the reaction mixture .
two hours later , the formed product was centrifuged and washed with ethanol to remove the unreacted rhodamine 6 g and silane .
the final particles were denoted as fs6 nanoparticles . for comparison , rhodamine 6g - doped silica shell - coated iron oxide nanoparticles
typically , 0.75 ml of s1 , 1.75 ml of h2o , 0.4 ml of ammonia , 12.5 ml of isopropyl alcohol , and 10 l teos were mixed together .
then it was stirred for 3 h. subsequently , 5 ml of rhodamine 6 g solution in isopropyl alcohol and 20 l of teos were added into the mixture . after stirring for 0.5 h , 5 ml of isopropyl alcohol , 2.5 ml of h2o , and 0.25 ml of ammonia was added dropwise into the reaction mixture simultaneously .
the reaction mixture was further stirred for 24 h. the final product was denoted as fs62 nanoparticles .
x - ray diffraction ( xrd ) pattern of the synthesized products were measured on an x pertpro philips x - ray diffractometer from 10 to 90. transmission electron microscopy ( tem ) was performed on a hitachi-600 transmission electron microscope . a nicolet nexus 670 fourier
transform infrared spectra ( ft - ir ) spectrometer was employed to determine the chemical composition of the synthesized composites in the range of 4000400 cm .
magnetic property of the final sample was measured at room temperature by a vibration sample magnetometer ( vsm , lakeshore 730 , america ) .
a rf-5301 pc fluorescence spectrophotometer was used to determine the photoluminescence ( pl ) spectra of this multifunctional nanomaterial .
iron oxide nanoparticles were prepared by adding ammonia to an aqueous solution of fe / fe at a 1:2 molar ratio .
. then the iron oxide nanoparticles were coated with rhodamine 6 g doped silica shell .
typically , 0.75 ml of s1 , 1.5 ml of h2o , 0.6 ml of ammonia , and 10 ml of isopropyl alcohol were mixed together under magnetic stirring .
subsequently , 5 ml of rhodamine 6 g solution in isopropyl alcohol and appropriate volume of htmos was added into the mixture . after stirring for 3.0 h ,
5 ml of isopropyl alcohol and 80 l of teos were added into the reaction mixture .
two hours later , the formed product was centrifuged and washed with ethanol to remove the unreacted rhodamine 6 g and silane .
the final particles were denoted as fs6 nanoparticles . for comparison , rhodamine 6g - doped silica shell - coated iron oxide nanoparticles
typically , 0.75 ml of s1 , 1.75 ml of h2o , 0.4 ml of ammonia , 12.5 ml of isopropyl alcohol , and 10 l teos were mixed together .
then it was stirred for 3 h. subsequently , 5 ml of rhodamine 6 g solution in isopropyl alcohol and 20 l of teos were added into the mixture . after stirring for 0.5 h , 5 ml of isopropyl alcohol , 2.5 ml of h2o , and 0.25 ml of ammonia was added dropwise into the reaction mixture simultaneously .
the reaction mixture was further stirred for 24 h. the final product was denoted as fs62 nanoparticles .
x - ray diffraction ( xrd ) pattern of the synthesized products were measured on an x pertpro philips x - ray diffractometer from 10 to 90. transmission electron microscopy ( tem ) was performed on a hitachi-600 transmission electron microscope . a nicolet nexus 670 fourier
transform infrared spectra ( ft - ir ) spectrometer was employed to determine the chemical composition of the synthesized composites in the range of 4000400 cm .
magnetic property of the final sample was measured at room temperature by a vibration sample magnetometer ( vsm , lakeshore 730 , america ) .
a rf-5301 pc fluorescence spectrophotometer was used to determine the photoluminescence ( pl ) spectra of this multifunctional nanomaterial .
1 . the peaks in the range between 30 and 70 indicated the prepared iron oxide nanocrystals have an inverse spinel structure . and
their average particle size was calculated to be about 10 nm by ( 3 1 1 ) peak .
the broad featureless peak , which was found at the low diffraction angle in fig .
xrd pattern of the fs6 nanoparticles figure 2shows the representative tem images of iron oxide nanoparticles and fs6 nanoparticles prepared under different conditions . as shown in fig . 2a
, the majority of the iron oxide nanoparticles were spherical with an average particle size around 10 nm , which was in agreement with the xrd result . as shown in fig .
2b and c , with the other preparation conditions remaining the same , the average particle size of the fs6 nanoparticles prepared by 10 and 40 l htmos correspond to 100 and 150 nm , respectively .
it indicated the thickness of the silica shell could be tuned by simply varying the initial amount of htmos .
but the silica shell was not very dense which may due to the long hydrophobic tail of htmos molecules .
it was also found that water volume play an important role in controlling the morphology of the final particles .
the volume of water used to prepare the particles in fig . 2c and d was 1.5 and 0.75 ml , respectively .
we can see that the final fs6 nanoparticles were all rather monodispersed , but the particles in fig .
this observation suggested that the particles tended to aggregate as the volume of water decreased .
so other measurements were still needed to prove the existence of rhodamine 6 g in the fs6 nanoparticles .
tem images forairon oxide nanoparticles , bfs6 particles prepared by 10 l htmos and 1.5 ml h2o , cfs6 prepared by particles 40 l htmos and 1.5 ml h2o , dfs6 particles prepared by 40 l htmos and 0.75 ml h2o figure 3 gives the ft - ir spectra of neat rhodamine 6 g , silica - coated magnetic particles ( denoted as fs nanoparticles ) , and fs6 nanoparticles .
the absorption bands for neat rhodamine 6 g   and fs nanoparticles   could be well resolved .
the ft - ir spectrum of fs6 nanoparticles was similar with that of the fs nanoparticles except one new peak at around 1,370 cm , which was marked by a black line in fig .
3 . according to the previous studies , these new peaks were associated with c 
but because of the confinement effects of sio2 shell which hindered most of the stretching and vibrational modes of the dye molecules , the other peaks of rhodamine 6 g are absent in fig .
so the entrapment of organic dye in the silica shell should be further confirmed by more experimental evidences .
ft - ir spectra of neat rhodamine 6 g , fs , and fs6 nanoparticles the emission spectra of fs62 and fs6 nanoparticles prepared by different volume of htmos were investigated and the results are shown in fig .
as observed , the pl intensity of fs62 nanoparticles was very weak ( line a ) , which demonstrated little rhodamine 6 g molecules were entrapped in the silica matrix .
lines b , c , and d showed the emission peaks of fs6 nanoparticles prepared by 10 , 20 , and 30 l of htmos , respectively .
their high pl intensity suggested the organic dye molecules can be entrapped in the silica matrix successfully by the hydrolysis of htmos .
furthermore , the maximum intensity of lines b , c , and d in fig . 4
this phenomenon indicated that the amount of organic dye doped in the silica shell was increased with the volume of htmos increasing .
so this data sustained the assumption that the driving force for the entrapment of organic dye molecules was the hydrophobic interaction between organic dye and htmos molecules .
furthermore , the pl spectrum of the final sample further confirmed the entrapment of organic dye in the final samples .
pl spectra ofafs62 nanoparticles and fs6 nanoparticles prepared by different volume of htmos , b10 l , c20 l , andd30 l ; ex = 520 nm , em = 560 nm the dye leakage behavior of fs6 nanoparticles in aqueous solution was also investigated . before every measurement
, the sample was washed with water and then resuspended in water to the original volume . as shown in fig .
5 , the pl intensity of the particles measured everyday showed no significant differences in 6 days .
it indicated most of the dye molecules were trapped within the nanoparticles and the optical property of the final samples was stable .
fluorescence intensity variation of fs6 nanoparticles immersed in water for 6 days figure 6shows the fluorescence microscopic images of the fs6 nanoparticles .
on the one hand , the particles were presented as bright dots which indicated the final samples were fluorescent . on the other hand ,
their green color was in agreement with the pl spectra because their emission wavelength was 560 nm .
confocal fluorescence image of the final fs6 nanoparticles figure 7 shows the magnetization hysteresis loops of the final samples measured at room temperature .
it is well known that iron oxide nanoparticles smaller than 20 nm are usually superparamagnetic at room temperature .
the mean size of the prepared iron oxide nanoparticles was about 10 nm , so our measurements were in agreement with this view .
the saturation magnetization ( ms ) of the final samples was about 6 emu / g .
in summary , a new method for preparing iron oxide nanoparticles coated with organic dye - doped silica shell was developed .
the preparation procedure was carried out in a bulk aqueous / isopropyl alcohol system at room temperature , which make it environmental friendly and low cost . in addition , the preparation procedure was relatively facile .
the characterization results by xrd , tem , ft - ir , vsm , pl spectra , and confocal fluorescence microscopy indicated that the final nanoparticles possessed magnetic and fluorescent properties simultaneously .
so this new method was efficient in preparing organic dye - doped silica shell - coated iron oxide nanoparticles .
in addition , we predict that this method can be applied to synthesis other fluorescent - magnetic nanoparticles .
the project was supported by the open subject foundation of key laboratory for magnetism and magnetic materials of moe , lanzhou university .
a new method for preparing magnetic iron oxide nanoparticles coated by organic dye - doped silica shell was developed in this article . 
 iron oxide nanoparticles were first coated with dye - impregnated silica shell by the hydrolysis of hexadecyltrimethoxysilane ( htmos ) which produced a hydrophobic core for the entrapment of organic dye molecules . 
 then , the particles were coated with a hydrophilic shell by the hydrolysis of tetraethylorthosilicate ( teos ) , which enabled water dispersal of the resulting nanoparticles . 
 the final product was characterized by x - ray diffraction , transmission electron microscopy , fourier transform infrared spectroscopy , photoluminescence spectroscopy , and vibration sample magnetometer . 
 all the characterization results proved the final samples possessed magnetic and fluorescent properties simultaneously . and 
 this new multifunctional nanomaterial possessed high photostability and minimal dye leakage .