Fluorescent Microparticles

Fluorescently labeled particles are useful as standards in flow cytometry, confocal laser scanning microscopy, and with light scattering instruments. They also have been used in environmental science as tracers in flow measurements of gases and liquids, such as laser Doppler anemometry (LDA), particle dynamic analysis (PDA), and particle image velocimetry (PIV).

Figure 1: Transmission electron microscopy (TEM) image of 470 nm PS-particles (reproduced with permission of Microparticles GmbH)

Melamine Resin Particles Sigma offers a new generation of monodisperse polymer microspheres (see Figure 1 ) based on melamine resin (MF). Melamine resin (MF) microspheres are manufactured by acid-catalyzed hydrothermal polycondensation of methylol melamines in a temperature range 70-100°C without any surfactants. By adjusting the pH value, the concentration of methylol melamine, and the reaction temperature, monodisperse particles with a predictable size between 0.5 to 1.5 mm can be produced in a one-pot synthesis. With excellent physical and chemical properties, melamine resin particles offer many advantages over other conventional polymer particles.

Physical and Chemical Properties of Melamine Resin Particles

• Density: 1.51g/cm ³
• Refractive Index: 1.68
• Excellent monodispersity (C.V. <3% ) and highly uniform spherical shape
• Hydrophilic surface
• High crosslinking density
• High thermostability up to 300°C
• Superior mechanical strength
• Stable and insoluble in acids and bases
• Extremely high stability in organic solvents, no swelling or shrinking upon contact with organic solvents
• Outstanding long-term stability in dispersions, no additives or stabilizers required
• Aqueous suspensions are stable to repeated freeze-thaw cycles
• Particles can be dried directly from their aqueous dispersions
• Free flowing powders of dried particles can be redispersed in any dispersing agent without agglomeration

Unmodified MF particles have a hydrophilic, charged surface due to the high density of polar triazine-amino and -imino groups. The surface functional groups (methylol groups, amino groups, ect.) allow covalent attachment of other ligands. For special applications, the MF particles can be modified by incorporation of other functionalities such as carboxyl groups. This increases possible surface derivatization such as chromophore or fluorophore labeling.

Micro particles based on melamine resin, FITC labeled

Micro particles based on melamine resin, nile blue labeled

Micro particles based on melamine resin, rhodamine B labeled

Fluorescently Labeled and Carboxylate-Modified Species

Fluorescent melamine resin microspheres can be prepared with variations in size, type of fluorochrome, and surface functional groups. Typical dyes used in MF fluorescent particles are:

• FITC (green fluorescence, l _Ex = 506 nm, l _Em = 529 nm)
  
  
• Rhodamine B (orange fluorescence, l _Ex = 560 nm, lEm = 584 nm)
  
  
• Nile Blue A (red fluorescence, l _Ex = 636 nm, l _Em = 686 nm)
  
  

Outstanding characteristics of fluorescent MF particles (see figure 2) are their narrow size distribution and an intense color and fluorescence. The homogeneously volume-stained particles have no leaching of the internally incorporated fluorochromes and they show high stability in organic solvents when comparable to white MF particles. Fluorescent MF beads are also available with carboxylate-modified surfaces containing a high density of functional groups (>0.1 mmole per gram of resin).

Figure 2: Fluorescence microscopy image of FITC-labeled MF particles (green) and Rhodamine B-labeled MF particles (red).

Micro particles based on melamine resin, carboxylate modified, FITC labeled

Micro particles based on melamine resin, carboxylate modified, nile blue labeled

Micro particles based on melamine resin, carboxylate modified, rhodamine B labeled

Melamine resin microspheres are available as white particles or with internally incorporated fluorescent labels. Both particle types are available with either unmodified or carboxylated surfaces.