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Electrical Double Layer-Induced Ion Surface Accumulation for Ultrasensitive Refractive Index Sensing with Nanostructured Porous Silicon Interferometers Stefano Mariani, Lucanos Marsilio Strambini, and Giuseppe Barillaro ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00650 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018
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Electrical Double Layer-Induced Ion Surface Accumulation for Ultrasensitive Refractive Index Sensing with Nanostructured Porous Silicon Interferometers
Stefano Mariani1, Lucanos Marsilio Strambini2, Giuseppe Barillaro1,2* 1
Dipartimento di Ingegneria dell’Informazione, Università di Pisa, via G. Caruso 16, 56122 Pisa,
Italy 2
Istituto di Elettronica e di Ingegneria dell’Informazione e delle Telecomunicazioni, Consiglio
Nazionale delle Ricerche, via G. Caruso 16, 56122 Pisa, Italy *
[email protected] Abstract Herein, we provide the first experimental evidence on the use of electrical double layer (EDL)induced accumulation of charged ions (using both Na+ and K+ ions in water as model) onto a negatively-charged nanostructured surface (e.g. thermally growth SiO2) – Ion Surface Accumulation, ISA – as a means of improving performance of nanostructured porous silicon (PSi) interferometers for optical refractometric applications. Nanostructured PSi interferometers are very promising optical platforms for refractive index sensing due to PSi huge specific surface (hundreds of m2 per gram) and low preparation cost (less than 0.01$ per 8’’silicon wafer), though they have shown poor resolution (R) and detection limit (DL) (in the order of 10-4– 10-5 RIU) compared to other plasmonic and photonic platforms (R and DL in the order of 10-7– 10-8 RIU). This can be ascribed to both low sensitivity and high noise floor of PSi interferometers when bulk refractive index variation of the solution infiltrating the nanopores either approaches or is below 10-4 RIU. Electrical double layer-induced ion surface accumulation (EDL-ISA) on oxidized PSi interferometers allows the interferometer output signal (spectral interferogram) to be impressively amplified at bulk refractive index variation below 10-4 RIU, increasing, in turn, sensitivity up to two ACS Paragon Plus Environment
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orders of magnitude and allowing reliable measurement of refractive index variations to be carried out with both DL and R of 10-7 RIU. This represents a 250-fold-improvement (at least) with respect to the state-of-the-art literature on PSi refractometers and pushes PSi interferometer performance to that of state-of-the-art ultrasensitive photonics/plasmonics refractive index platforms.
Keywords: refractive index sensor, differential refractometer, nanostructured porous silicon, interferometer, electrical double layer, ion surface accumulation
Recently, both scientific and industrial communities have paid increasingly attention to ultrasensitive refractive index (RI) measurements for a broad range of applications, from pharmaceuticals, medicine and chemicals, to flavors, beverages and food1-10. Refractive index optical sensors based on plasmonic4,5 and photonic micro and nanostructures6-10 have recently attracted increased attention for the realization of ultrasensitive and ultra-compact refractometers. Either evanescent field (e.g. surface plasmon resonance11-13, long period grating fibers6,14,15), or resonant (e.g. localized SPR16-21, ring resonators22,23) and interference (e.g. interferometers24-26, photonic crystals,27,28) effects between electromagnetic field and matter of interest have been exploited to quantify refractive index variations by increasing light-matter interaction at the micro and/or nanoscale. An increased light-matter interaction leads, indeed, to an augmented sensitivity (S) and, in turn, to a higher resolution (R) that ultimately gives the minimum variation of refractive index that can be resolved above the noise floor (σ), also known as detection limit (DL=3σ/S)29. On the other hand, an augmented sensitivity to signal might also result in an increased cross-sensitivity (cS) toward disturbs (e.g. thermal, light, and fluidic fluctuations), which eventually leads to an augmented noise floor and, in turn, to poorer resolution and detection limit. The latter might explain the limited improvement on the performance of ultrasensitive refractive index optical sensors achieved over the last two decades, for which best R and/or DL values are stuck between 10-7 and 10-8 RIU in spite of refractometer architectures, operating principles, readout ACS Paragon Plus Environment
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methods, and amplification strategies that have been exploited to date3. This is evident if we focus the attention on the numerous plasmonic refractometers that have been recently proposed with the aim of enhancing refractive index sensing performance, which include magnetoplasmonic antennas exploiting control of the phase of light to boost sensitivity (S=230nm/RIU)11; planar plasmonic interferometers consisting of nanoscale grooves and slits milled in a metal film to form two-arm, three-beam interferometers (R=3×10−7 RIU)26; metal-coated optical fibers enabling excitation of surface plasmon polaritons (R=10−8RIU)12; gold mushrooms arrays enabling excitation of Fano resonance (R=10-4 RIU)17; hyperbolic metamaterial supporting highly confined bulk plasmon guided modes over a broad wavelength range (S=30000 nm/RIU)13; plasmonic chiral nanoparticles with engineered dispersion and shape
(S=1091 nm/RIU)19, capped gold nanoslit arrays with
extremely sharp asymmetric resonances for a transverse magnetic-polarized wave (R=4×10-6 RIU)30. Among high sensitivity photonic platforms, nanostructured porous silicon (PSi) interferometers have been proposed over the last two decades as ultrasensitive label-free biosensors31-36 able to specifically detect pM to fM concentration of bioanalytes32 upon functionalization of the PSi surface with suitable bioreceptors. Chief advantages of PSi interferometers over other photonic and plasmonic platforms are: 1) a huge specific surface (hundreds of m2 per gram) that allows a massive number of molecules to be accommodated, thus increasing interaction between electromagnetic field and molecules of interest and improving, in turn, sensitivity; 2) a straightforward preparation (one single non-patterned electrochemical etching step) that allows PSi preparation cost to be lowered down to 0.01$ per 8’’ siliconwafer31. A further advantage of PSi with respect to other miniaturized photonic platforms is its unique intrinsic capability of filtering out anything with dimension either larger or comparable with the pore size, thus reducing possible interferences and artifacts in real-field analysis. In spite of PSi great premises, PSi interferometer performance for refractometric applications are several orders of magnitude far from state-of-the-art photonic and plasmonic structures, these latter being able to discriminate refractive index changes with resolution ACS Paragon Plus Environment
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and detection limit in the range of 10-7– 10-8 RIU. The poorer performance of PSi interferometers can be ascribed to a lower sensitivity and a higher noise floor when bulk refractive index variation of the solution infiltrating the pores is below 10-4 RIU. Here, we report on ultrasensitive refractive index sensing with PSi interferometers via electrical double layer (EDL)-induced accumulation of charged ions (e.g. Na+ and K+ ions in water) onto a negatively-charged nanostructured surface (i.e. thermally-grown silicon dioxide), namely Ion Surface Accumulation (ISA). A solid experimental characterization coupled with an effective theoretical modeling in NaCl aqueous solutions shows that EDL-ISA tremendously amplifies the output signal (spectral interferogram) of oxidized PSi interferometer when bulk refractive index variation of the solution infiltrating the pores is below 10-4 RIU (i.e. NaCl (w/w) 6×10-5 RIU), which is best fitted by the following linear equation: IAW-IAW0 = 617∙∆nsol (R2 = 0.999); remarkably, a non-linear relationship (Figure 3a, blue trace) is ACS Paragon Plus Environment
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recorded at the lower NaCl concentrations (∆nsol0.1% w/w). Remarkably, we found that the noise level σNaCl remains roughly constant regardless of the NaCl concentration, namely σNaCl=0.004 a.u. (average value) for %NaCl ˂0.1% w/w and σNaCl=0.006 a.u. (average value) for %NaCl≥0.1%w/w, and close to the noise floor in DIW σIAW0=0.003 a.u.. From these values, a minimum resolution of about 10-7 RIU, i.e. of the same order of the detection limit, is obtained at %NaCl0.01% w/w.
Sensitivity amplification by Electrical Double Layer (EDL)-induced ion accumulation at the PSi interferometer surface The two different trends apparent in the PSi interferometer calibration curve (Figure 3a) at lower and higher NaCl concentration can be explained in terms of surface (Figure 3c) and bulk (Figure 3d) refractive index variation effect, respectively. Generally speaking, in the presence of a solution infiltrating the pores, both molecule adsorption on the pore surface (surface effect) and molecules floating in the solution volume (bulk effect) contribute to set the effective refractive index value, neff, of the PSi layer. Although both the effects occur simultaneously, at the lower NaCl concentrations (below 0.1% w/w, i.e. ∆nsol=6.3×10-5 RIU) surface effects due to adsorption of Na+ or Naδ+ (solvated) ions on the negatively charged PSi oxidized surface dominates over bulk effects due to ions present in the solution infiltrating the pores. In fact, at NaCl concentrations below 0.1% the refractive index variation of the solution infiltrating the pores is too small, namely ~0.1% w/w (see inset). In the inset in b) is reported the linear trend for %NaCl> ~0.1% (red trace). Solid lines and gray area same as in a). c) Sketch of EDL-ISA adsorption of Na+ (or Naδ+) ions onto the negatively charged oxidized inner surface of PSi interferometers upon infiltration with DIW solutions at %NaCl ˂ 0.1% w/w, yielding up to a 250-fold amplification in sensitivity (surface effect). d) Sketch of Na+ ions infiltration inside the nanopores at %NaCl> 0.1% w/w, leading to bulk refractive index variation of the solutions infiltrated in the nanopores (volume effect). e) Time-resolved IAW-IAW0 signal upon infiltration of PSi interferometers with DIW solutions with NaCl concentration of 0.0001% w/w, corresponding to a refractive index variation of the solution (with respect to DIW) ∆nsol= 6.3×10-8 RIU (black dots represent experimental raw data, solid blue line represents filtered data, solid red line depicts NaCl solution injection time with respect to DIW injection). f) Time-resolved IAW-IAW0signal upon infiltration of PSi interferometers with DIW solutions with NaCl concentration of 2, 5, and 10% w/w, corresponding to refractive index variation of the solutions (with respect to DIW) ∆nsol of 1.16×10-3, 2.94×10-3, and 6.05×10-3 RIU, respectively (black solid line represent experimental raw data, red solid line represent NaCl solution injections with respect to DIW). ACS Paragon Plus Environment
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Figure 4. Ultrasensitive refractive index measurements of KCl aqueous solutions with nanostructured PSi interferometers. a) Experimental calibration curve (solid dots) in log-log scale of the proposed PSi interferometers operating as optical refractive index sensors showing the output signal IAW-IAW0 (y-axis) plotted versus both %KCl (w/w) concentration (top x-axis) and equivalent refractive index variation ∆nsol (bottom x-axis) of solutions infiltrated in the nanopores. Solid lines represent best-fitting curves of experimental data at the lower (%KCl˂0.1% w/w) and higher (%KCl>0.1% w/w) KCl concentrations. Gray area represents the region within which the IAW-IAW0 is smaller than 3 times the noise level σIAW0 (i.e. 3σIAW0= 0.010 a.u.). b) Representation of the experimental calibration curve in a) on a linear scale to highlight the non-linear behavior of the calibration curve at %KCl˂ 0.1% w/w due to EDL-ISA of K+ ions on the oxidized PSi surface; the calibration curve becomes linear for %KCl>0.1% w/w (see inset). In the inset in b) is reported the linear trend for %KCl> 0.1% (red trace). Error bars of each experimental point of the calibration curve are estimated taking into account the IAW signal fluctuations over the last two minutes of each injection.
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BULK EDL-ISA
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