Silicon Nanowire Based Single-Molecule SERS Sensor

Transcription

1 Supporting informtion Silicon Nnowire Bsed Single-Molecule SERS Sensor Hui Wng, Xuemei Hn, Xuemei Ou, Chun-Sing Lee, Xiohong Zhng* nd Shuit-Tong Lee S1, A series of Si nnowires coted with compct ggregtes of silver nnoprticles (AgNPs) hve een controllly fricted. AgNPs cn e controllly deposited on silicon nnowires (SiNWs) through djusting the concentrtion of AgNO 3 nd cetyltrimethyl mmonium romide (CTAB). The surfce-enhnced Rmn scttering (SERS) enhncement is gretly ffected y the density of AgNPs. It ws found tht when the formed AgNPs re smller (<20 nm), stnding lone (Figure S1) or hving few ggregtions (Figure S1), their enhncement effect is lower nd the resulting SERS spectrum of R6G is wek (Figures S2 nd S2). When the AgNPs re out 34 nm, nd with n verge interprticle spcing of < 7 nm (Figure S1c), SERS spectr cn e collected with fine spectroscopic fingerprinting (Figure S2c). However, while AgNPs re forming continuous film (Figure S1d), the sensitivity nd resolution of the SERS spectr re decresed y the presence of continuum ckground (Figure S2d). c d Figure S1. TEM imges of AgNPs modified SiNWs synthesized y dding X ml of 1 mm AgNO 3 solution to l ml of SiNWs-CTAB dispersion. ) X=0.5; ) X=1.0; c) X=3.0; d) X=4.0. 1

2 Intensity (.u.) d c Rmn shift (cm -1 ) Figure S2. SERS spectr recorded from single AgNPs@SiNW. Spectr,, c, nd d were collected respectively from smples,, c, nd d s shown in Figure S1. The concentrtion of R6G is M. S2, Quntittive evlution of the enhncement fctor. The SERS enhncement fctor (EF) ws clculted with EF SERS =I SM-SERS N ulk /I ulk N SM-SERS, where I SM-SERS nd I ulk re the intensities of the nd t 1648 cm -1 from the AgNPs@SiNW nd tht of ulk solid smple, respectively. N SM-SERS nd N ulk re the numer of molecules on the surfce of the re irrdited y the lser nd in the ulk with the volume of irrdition. The experiment ws performed under the sme lser power (0.05 mw) nd cquisition time (10 s). The wek incident lser power nd short cquisition time were used to void photoleching. 1) Determining the I ulk nd N ulk nd clculting N ulk /I ulk. Since the Rmn spectrum of R6G solution with high concentrtion is overlpped y fluorescence, the I ulk nd N ulk were mesured with R6G molecule thin film. 0.05mL R6G solution ( M) ws dropped into circulr glss tnk (ø=8.5 mm). After evporting the wter in ir, the ottom of the glss tnk ws covered y thin R6G film. 50 ojective (NA= 0.5, effective lser focus cross section ws 1.3 μm in dimeter) ws used to collect Rmn spectrum. The intensity ( counts) t 1648 cm -1 ws otined fter Gussin fitting the spectrum with the Wire 3.0 pek fit tools of the Rmn spectroscope. The totl molecule numer on the ottom of the glss tnk cn e otined: N totl = C V NA= = (NA is the Avogdro s Numer) The molecule numer of the ulk in the volume of irrdition is: N ulk =(A ex /A totl ) N totl = (A ex nd A totl re the re of the lser focus nd the glss tnk) Then, N ulk /I ulk = ) Determining the I SM-SERS nd N SM-SERS nd clculting I SM-SERS /N SM-SERS. 100 Rmn spectr of M R6G solution were selected (N SM-SERS =1.18) to clculte the SM-SERS intensity, in which the spectrum of R6G is definite. The verge, mximum nd minimum SM-SERS intensity re 123.3, nd 11.2 counts, respectively. Then (I SM-SERS /N SM-SERS ) verge = ; (I SM-SERS /N SM-SERS ) mx = ; nd (I SM-SERS /N SM-SERS ) min = Bsed on the ove nlysis, the verge EF SERS for the single-molecule detection in our experiment is , nd the mximum nd minimum re nd , respectively. 2

3 Intensity (.u.) S3, Single system hs higher SERS sensitivity nd etter stility thn AgNPs on Si wfer system Wfer Rmn shift (cm -1 ) Figure S3. SERS spectr of M R6G solution collected respectively from Ag NPs-deposited Si wfer nd single AgNPs@SiNW under 50 ojective (NA=0.5) with the sme experiment conditions. Morphology of AgNPs on Si wfer is shown in the Figure S4. AgNPs coted on Si wfer hve n verge dimeter of 44 nm in the rnge of nm. Its prticle density is 180/ m 2 (Figure S4). AgNPs on the SiNWs hve n verge dimeter of 34 nm (in the rnge of nm). Its liner prticle density is 132.5/ m (Figure S4).The confocl lser spot of the 50 ojective (NA=0.5) is 1.3 m in dimeter, which mens the re of the lser spot will e 1.33 m 2. Bsed on this, the numer of AgNPs within the lser spot is nd for Si wfer nd SiNWs, respectively. Figure S4. ) SEM nd ) TEM imges of AgNPs on Si wfer nd SiNWs, respectively. Tle 1. The numer of AgNPs nd the intensity t 1648 cm -1 from AgNPs-coted Si wfer nd SiNWs. AgNPs Dimeter (nm) Density Are of the lser spot Numer I 1648 On Si Wfer 44 nm (27-56 nm) 180/ m m On Si nnowires 34 nm (25-53 nm) 132.5/ m 1.3 m Aove results show tht the nd intensity t 1648 cm -1 (romtic stretching virtions) collected from AgNPs@SiNW is out 10 times stronger thn tht from AgNPs-coted Si wfer (Figure S3), lthough the numer of Ag nnoprticles on SiNW within the smpling proe re less thn tht on the Si wfer (Tle 1). In 3

4 Intensity (.u.) Asornce (Ar.Unit) ddition, AgNPs on Si wfer system show unstle morphology fter eing immersed in D. I. wter for 24 hours. Figure S5. Morphology chnge of AgNPs on Si wfer efore ) nd fter ) eing immersed in D. I. wter. S4, AgNPs@SiNWs hs stronger photosorption nd Rmn scttering intensity thn ggregted AgNPs colloids SiNWs 0.1 AgNPs@SiNWs AgNPs Wvelengh (nm) Figure S6. UV Vis sorption spectr of SiNWs, AgNPs@SiNWs, nd AgNPs (mesured y HiTAchi U-3010 spectrometer) SiNWs AgNPs@SiNWs AgNPs Rmn shift (cm -1 ) Figure S7. Rmn spectr collected from SiNWs, AgNPs@SiNWs, nd AgNPs. The AgNPs@SiNWs show stronger Rmn intensity thn AgNPs. 4

5 S5, Preprtion of SiNWs The SiNWs were synthesized using high-temperture tue furnce, nd this set up hd een descried previously. An Al 2 O 3 ot with mixture of 3.00 g SiO (Aldrich, 325mesh, 99.9%) nd 0.50 g Sn ws put in the center of n lumin tue, which ws then mounted in the furnce. The system ws pumped to mr efore heted to 1320 o C t 40 o C/min then mixture of rgon (95%) nd hydrogen (5%) t 350 mr ws kept flowing t flow rte of 10 sccm. After out 7 h growth, the furnce ws cooled to room temperture t 40 o C /min. A drk yellow product out 0.5 grm ws otined on the silicon wfer nd the inside wll of the lumin tue t o C zone of the downstrem end, nd the product yield cn e incresed through prolonging the rection time. The product ws checked with scnning electron microscopy (SEM; Philips XL 30 FEG, Hollnd nd X-ry diffrction (XRD: Siemens D500). Some of the smple ws dispersed in ethnol nd drop of the dispersion ws put on cron-coted copper TEM smple grid for exmintion y trnsmission electron microscopy (TEM; Philips, CM20, operted t 200kV, Hollnd. To record the room-temperture photoluminescence spectrum, the smple ws fixed etween two pieces of crystl pltes, nd 3941C-M1BB/Spitfire FF-1K/OPA-800CF-0.5 femto-second lser (300nm) ws used s the excittion source. S6, Spectrum linking cn only e found in single molecule detecting. Three-dimensionl Rmn spectrum of single nd multi-molecule hd een compred s following: the spectrum of single molecule ( M R6G, 100 ojective, 1.17 molecules in the confocl lser spot) oviously showed linking, while the multi-molecule ( nd M R6G, 100 ojective, 11.7 nd 117 molecules in the confocl lser spot) showed grdul chnge. The result is consistent with the litertures.. Single molecule. 11 moleculr c. 117 moleculr Figure S8. Three-dimensionl Rmn spectrum of single () molecule, 11 () nd 117 (c) moleculr. The 3-D spectrum composed 100 spectr tht were collected in 40 minutes, respectively. 5