A simple method used to construct microparticles of porous Si photonic crystals that display highly resolved and easily controlled optical diffraction peaks is described. The spectral diffraction peaks can act as digits in an encoding scheme, and the possibility of generating particle libraries with >106 distinct codes is demonstrated (see Figure).

A particle-based multiplexed DNA assay based on encoded porous SiO2 photonic crystal disks is demonstrated. A “spectral barcode” is generated by electrochemical etch of a single-crystal silicon wafer using a programmed current-time waveform. A lithographic procedure is used to isolate cylindrical microparticles 25 microns in diameter and 10 microns thick, which are then oxidized, modified with a silane linker, and conjugated to various amino functionalized oligonucleotide probes via cyanuric chloride. It is shown that the particles can be decoded based on their reflectivity spectra, and that a multiple analyte assay can be performed in a single sample with a modified fluorescence microscope. The homogeneity of the reflectivity and fluorescence spectra, both within and across the microparticles is also reported.

Experiments in biomedical research require the need to screen for large numbers of analytes in very small volume samples. This need has led to the development of two main classes of multiplexing technologies: microarrays and encoded microparticles [1]. While microarrays separate the individual assays in a fixed, spatially differentiated array, the individual assays in an encoded microparticle system are identified by placing a specific label on each microparticle. Specific labels such as fluorescent molecular tags [1], reflective metal stripes [2], and photonic structures [3,4] have been used. The present work is based on onedimensional photonic crystals, microfabricated from thin films of porous silicon. These materials can be functionalized with biomolecules and their use in biomolecular screening assays have been demonstrated [5]. The approach uses photolithography to define the particle dimensions in the xand y-coordinates [6]. The z-coordinate contains the encoding information, defined using a temporally modulated current waveform during electrochemical etch.The photonic crystal encoding information is produced by anodically etching p-type, boron-doped, (100) oriented silicon with <1 mΩ cm resisitivity (Siltronix, Inc.) in a solution of 48% aqueous HF:ethanol (3 :1 by volume). A computer-generated waveform containing the encoding information is used to control the electrochemical reaction. The encoding technique is described in detail in previous work [4]. Figure 1 summarizes the photolithographic process used to define the xand y-dimensions of the particle. First, an aluminum layer is sputter-coated onto the surface of the porous silicon film surface at a nominal thickness of 40 nm. The substrate is then heated at 200 °C for 3 minutes and SU8-25 photoresist (Microchem) is spin-coated on the aluminum layer, producing a ~20 µm-thick film. The function of the aluminum layer is to allow for clean removal of the photoresist from the porous silicon particles at the end of the process. The substrate is soft-baked on a hot plate in two steps: 65 °C for 3 min followed by 95 °C for 7 min. After the soft-bake, the photoresist-covered substrate is exposed to UV light (λ = 365-405 nm) for 25 s, through a contact mask. The mask consists of a square array of circles, each 25 µm in diameter with a pitch of 35 µm. The mask is aligned to the center of the porous silicon sample. This reduces spectral non-uniformity of the particles by eliminating the edges of the film from the microfabrication process. The porosity at the edges of the film varies due to restricted electrolyte diffusion and nonuniform current density in the vicinity of the o-ring seal during the electrochemical etch. The UV-exposed sample is then baked at 65 °C for 1 min and at 95 °C for 1 min. The substrate is then submerged in SU8 developer (Microchem) for 4 min. At this point the sample consists of an array of SU8 features, 25 µm in diameter, bonded to an alu-A method to microfabricate micron-scale freestanding porous silicon photonic crystal particles is d...

Monodisperse, micrometer‐sized polymer beads can be imparted with a porous nanostructure by softening them into a porous silicon photonic crystal template. Infiltration of the polymer and replication of the photonic spectral properties is verified by optical reflectivity measurements of the beads as they infuse into the nanostructure. The polymer beads can be removed from the porous Si template by dissolution of the template with a basic solution. This treatment destroys the photonic properties, although scanning electron microscopy (SEM) reveals that some nanotexturing remains in the polymer beads. This simple, solvent‐free process provides a means to produce nanotextured particles of controllable size and shape. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)