Researchers recently created porous silicon mirrors and showed that these nanostructures enhance the signal from fluorescent molecules. Variations of the method could be used for the detection of low concentrations of biological molecules and for other biosensing applications.“Following the original idea of my French collaborator, professor Csilla Gergely from Université Montpellier II, the structures are used as optical amplifiers of fluorescent signals,” explained team member and Universidad Autónoma de San Luis Potosì physics professor Elìas Pérez. Besides Pérez and co-workers at the institution in Mexico, others on the team were from Universidad Nacional Autónoma de México in Temixco and from Université Montpellier II in France. The silicon mirror concept, recalled Pérez, arose from a series of discussions among the group members, who had expertise in the various areas that would be needed to make the scheme work. Researchers at the second institution, for example, did the fabrication.The idea was to use the crystalline nature of silicon to produce porous structures carefully fabricated to be the right size to trap fluorescent molecules. Fabricating the surface of the silicon so that it was highly reflective across the appropriate wavelength range would increase the excitation and enhance the emission, which are both driven by the efficient reflection of light. This boost would make it possible to detect faint fluorescent signals. The biocompatible features of the silicon would make the nanostructure potentially suitable for use as a sensor in living systems.This graph shows the fluorescence emission of the fluorescein-5 maleimide molecules which were deposited on the silane-functionalized p-type porous silicon mirrors. The numbers M45, M5 and M70 refer to samples fabricated under various conditions and with various resulting pore sizes and size distributions. As can be seen, the intensity is greatly enhanced if the pores have the right characteristics.In demonstrating the strategy, the researchers electrochemically etched p-type silicon wafers, varying the time and current density to produce pores that differed in size and density. They started with one current density, such as 5 mA/cm2 of silicon and then changed to 45 mA/cm2. They repeated this sequence 21 times, creating bands of alternating high — 2.83 — and low — 1.65 — refractive indices.They fabricated three types of samples, with the first layer having a current density of 5, 45 or 70 mA/m2. The second layer of all of them had a current density of 45 mA/m2. Finally, they bound the molecule mercaptopropyltrimethoxysilane (MPTS) to the surface through a chemical reaction. That step was necessary, the researchers said, because the group had chosen to functionalize the surface using fluorescein-5-maleimide molecules as a molecular probe. The MPTS chemically attached the fluorescein on the surface, assuring confinement.Judging from atomic force microscopy images collected with a Digital Instruments system, the lowest current density samples had pore diameters under the 20-nm probe tip resolution. Those samples in the middle had a homogeneous surface with a pore size of 250 ±60 nm. The highest current density samples had a varied surface and a wide range of pore sizes. Measurements of porosity were 42, 80 and 90 percent for the three current densities, with the lowest density being the least porous.Atomic force microscopy surface images show porous silicon mirrors (bottom) and their pore distributions (top) for samples fabricated with a 70 (M70) and 45 (M45) mA/cm2 current density during electrochemical etching. The highly reflective surface enhances the fluorescent signal of molecules trapped within the pores. Images courtesy of Elìas Pérez, Universidad Autónoma de San Luis Potosì.Once the fabrication was completed and the fluorophore was present on the surface, the researchers measured the reflectance spectra using a Shimadzu ultraviolet-visible-infrared spectrophotometer. They found that the mirrors produced different effects. Those manufactured with the two extremes of current density both exhibited a blueshift as compared with freshly cleaved and unfunctionalized samples. The middle density mirror, on the other hand, had a redshift of 85 nm.This difference, the researchers believe, can be attributed to a match between the molecules and the pore size. The dimensional agreement made it easier to confine the molecules within the pores and thereby enhance the effect. Also, the mirror’s reflection range, 468 to 540 nm, conveniently covered both the 491-nm excitation and the 521-nm emission peaks of the fluorophore.As a final step, the ability of the mirror to detect trace amounts of molecules was tested. The group prepared the silicon mirrors and exposed them to a 1.2-mM concentration solution of the fluorescein-bearing molecular probe. Neither the high nor low current density mirrors showed a detectable response at 521 nm. The middle current density, on the other hand, demonstrated significant emission, with several times the intensity of the others.“We were surprised [about the results] because we expected the same effect for the three samples,” Pérez said.However, he added that after considering the nature of the surfaces involved, they concluded that the middle density produced pores that were just right. The lowest density had pores that were too small to allow the efficient entry of molecules, and the highest density had pores that were too big for effective confinement. The work was published in the Sept. 17 issue of Applied Physics Letters.Pérez noted that the technique offers a significant advantage in that the mirror nanostructures can be produced inexpensively. There is no need for a more costly technique, such as laser ablation. Another plus is the simplicity of the processing, which, among other things, makes it easier to adjust the fabrication parameters to produce pores of the appropriate size.Members of the group have started using the structures in experiments with biological molecules and will be reporting results soon. Their hope is that eventually it will be possible to detect a single molecule with the nanostructures. However, before that happens, the manufacturing process may have to be changed and tightened. “These structures, as made up until now, are limited due to their large distribution of pore sizes,” Pérez said.Contact: Elìas Pérez; Instituto de Fisica, Universidad Autónoma de San Luis Potosì, Alvaro Obregon, Mexico; e-mail: elias@ifisica.uaslp.mx.