ARTICLES 8150 -log([PSAl(g/ml d 1,300 品 02000400060008,000 Time(s) Modification time(min) e Figure 1 Nanowire sensor arrays and detector properties. (a)Optical image(top)of a nanowire device array. the white lines correspond to the silicon tride passivated metal electrodes that connect to individual nanowire devices. the red rectangle highlights one of the repeated (vertical) regions where the nanowire devices are formed (see Supplementary Fig. l online for high resolution images of devices). The position of the microfluidic channel used to deliver has a total size of 6 m a The schematic(bottom)shows details of metal electrodes (golden lines)connecting nanowires (blue lines)in this region with orientation rotated 90%relative e to red rectangle. (b) Schematic showing two nanowire devices, I and 2, within an array, where the nanowires are modified with different (l, green; 2, red) antibody receptors. A cancer marker protein that binds specifically to its receptor (on nanowire- l)will produce a conductance change characteristic of he surface charge of the protein only on nanowire-l. (c)Change in conductance versus concentration of Psa for a p-type silicon nanowire modified with PSA-Abl receptor Inset: Conductance-versus-time data recorded after alternate delivery of Psa and pure buffer solutions; the psa concentrations were 0.9 ng/ml, 9 pg/ml, 0.9 pg/ml and 90 fg/ml, respectively. The buffer solutions used in all measurements were 1 uM phosphate(potassium salt)containing 2 HM KCl, pH=7.4.(d)Conductance-versus-time data recorded for a PSA-Abl-modified p-type silicon nanowire after alternate delivery of the following o protein and pure buffer solutions: (1)9 pg/ml PSA, (2)0.9 pg/ml PSA, (3)0.9 pg/ml PSA and 10 Hg/ml BSA, (4)10 ug/ml BSA and(5)9 PSA (e)Thickness dependence(red curve)of aldehyde silane layer on the sinw surfaces extracted from AFM measurements after different modification time the aldehyde propyltrimethoxysilane, and sensitivity dependence(blue curve) of detection of l ng/ml of PsA, after different modification time using a p-type SiNW device electrical contacts are formed in parallel by photolithography and The sensitivity limits of our silicon-nanowire devices were first metal deposition steps. In addition, different receptors can be printed determined by measuring conductance changes as the solution con- on the nanowire device array to allow selective multiplexed detection centration of PSa was varied, using devices modified with mAbs for (Fig. 1b). Selective binding of cancer marker proteins to surface-linked PSA(Abl). Representative time-dependent data(inset, Fig. lc)show a mAbs should produce a conductance change in the corresponding well-defined conductance increase and subsequent return to baseline receptor-modified silicon-nanowire device but not in devices lacking when PSA solution and pure buffer, respectively, are alternately he specific antibody receptor. In the case of a p-type(boron-doped) delivered through a microfluidic channel to the devices. a plot of silicon nanowire, applying a positive gate voltage depletes carriers and these data(Fig. lc)shows that the conductance change is directly reduces the conductance, whereas applying a negative gate voltage proportional to the solution PSA concentration for values from leads to an accumulation of carriers and an increase in conductance 5 ng/ml down to 90 fg/ml (the opposite effect occurs in n-type semiconductors). The depen- There are several key features of these data. First, the reversibility of dence of the conductance on gate voltage makes field-effect transistors the conductance changes demonstrates that nonspecific, irreversible natural candidates for electrically based sensing since the electric field protein binding does not occur to a measurable extent on the devices resulting from binding of a charged species to the gate dielectric is Second, the increases in conductance with PSa binding to the analogous to applying a voltage using a gate electrode. Thus, the Abl-linked, p-type nanowire devices are consistent with binding of onductance of a p-silicon nanowire will increase(decrease) when a a protein with negative overall charge, as expected from the pl of PSA, protein with negative(positive)surface charge binds to the antibody 6.8(ref. 24), and the pH, 7.4, of our experiments. Third, the receptor, whereas the opposite response should be observed for an show that direct, label-free detection of PSA is routinely n-type(phosphorus-doped) silicon nanowire with a signal-to-noise ratio >3 for concentrations down to ATURE BIOTECHNOLOGY VOLUME 23 NUMBER 10 OCTOBER 2005 1295electrical contacts are formed in parallel by photolithography and metal deposition steps. In addition, different receptors can be printed on the nanowire device array to allow selective multiplexed detection (Fig. 1b). Selective binding of cancer marker proteins to surface-linked mAbs should produce a conductance change in the corresponding receptor-modified silicon-nanowire device but not in devices lacking the specific antibody receptor. In the case of a p-type (boron-doped) silicon nanowire, applying a positive gate voltage depletes carriers and reduces the conductance, whereas applying a negative gate voltage leads to an accumulation of carriers and an increase in conductance (the opposite effect occurs in n-type semiconductors). The depen￾dence of the conductance on gate voltage makes field-effect transistors natural candidates for electrically based sensing since the electric field resulting from binding of a charged species to the gate dielectric is analogous to applying a voltage using a gate electrode. Thus, the conductance of a p-silicon nanowire will increase (decrease) when a protein with negative (positive) surface charge binds to the antibody receptor, whereas the opposite response should be observed for an n-type (phosphorus-doped) silicon nanowire18. The sensitivity limits of our silicon-nanowire devices were first determined by measuring conductance changes as the solution con￾centration of PSA was varied, using devices modified with mAbs for PSA (Ab1). Representative time-dependent data (inset, Fig. 1c) show a well-defined conductance increase and subsequent return to baseline when PSA solution and pure buffer, respectively, are alternately delivered through a microfluidic channel to the devices. A plot of these data (Fig. 1c) shows that the conductance change is directly proportional to the solution PSA concentration for values from B5 ng/ml down to 90 fg/ml. There are several key features of these data. First, the reversibility of the conductance changes demonstrates that nonspecific, irreversible protein binding does not occur to a measurable extent on the devices. Second, the increases in conductance with PSA binding to the Ab1-linked, p-type nanowire devices are consistent with binding of a protein with negative overall charge, as expected from the pI of PSA, 6.8 (ref. 24), and the pH, 7.4, of our experiments. Third, these data show that direct, label-free detection of PSA is routinely achieved with a signal-to-noise ratio 43 for concentrations down to 75 fg/ml 2 2 1 1 200 1.35 1.25 1.15 1.05 0 2,000 4,000 6,000 Time (s) Time (s) Modification time (min) Layer thickness (nm) Conductance (µS) 8,000 150 100 1,300 1,250 1,200 200 150 100 50 20 40 Sensitivity (∆ nS) 60 80 100 120 0 2 4 6 0 0 0 1 2 3 4 5 2,000 4,000 6,000 8,000 50 ∆Conductance (nS) Conductance (nS) 0 9 10 11 –log ([PSA] (g/ml)) 12 13 14 a d e b c Figure 1 Nanowire sensor arrays and detector properties. (a) Optical image (top) of a nanowire device array. The white lines correspond to the silicon nitride passivated metal electrodes that connect to individual nanowire devices. The red rectangle highlights one of the repeated (vertical) regions where the nanowire devices are formed (see Supplementary Fig. 1 online for high resolution images of devices). The position of the microfluidic channel used to deliver sample is highlighted by the dashed white rectangle and has a total size of 6 mm
500 mm, length
width. The image field is 8 mm
1.2 mm. The schematic (bottom) shows details of metal electrodes (golden lines) connecting nanowires (blue lines) in this region with orientation rotated 901 relative to red rectangle. (b) Schematic showing two nanowire devices, 1 and 2, within an array, where the nanowires are modified with different (1, green; 2, red) antibody receptors. A cancer marker protein that binds specifically to its receptor (on nanowire-1) will produce a conductance change characteristic of the surface charge of the protein only on nanowire-1. (c) Change in conductance versus concentration of PSA for a p-type silicon nanowire modified with PSA-Ab1 receptor. Inset: Conductance-versus-time data recorded after alternate delivery of PSA and pure buffer solutions; the PSA concentrations were 0.9 ng/ml, 9 pg/ml, 0.9 pg/ml and 90 fg/ml, respectively. The buffer solutions used in all measurements were 1 mM phosphate (potassium salt) containing 2 mM KCl, pH ¼ 7.4. (d) Conductance-versus-time data recorded for a PSA-Ab1-modified p-type silicon nanowire after alternate delivery of the following protein and pure buffer solutions: (1) 9 pg/ml PSA, (2) 0.9 pg/ml PSA, (3) 0.9 pg/ml PSA and 10 mg/ml BSA, (4) 10 mg/ml BSA and (5) 9 pg/ml PSA. (e) Thickness dependence (red curve) of aldehyde silane layer on the SiNW surfaces extracted from AFM measurements after different modification time of the aldehyde propyltrimethoxysilane, and sensitivity dependence (blue curve) of detection of 1 ng/ml of PSA, after different modification time using a p-type SiNW device. NATURE BIOTECHNOLOGY VOLUME 23 NUMBER 10 OCTOBER 2005 1295 ARTICLES © 2005 Nature Publishing Group http://www.nature.com/naturebiotechnology