Water-soluble NIR fluorescent probes based on squaraine and their application for protein labeling

Anal Sci. 2008 Feb;24(2):213-7. doi: 10.2116/analsci.24.213.


Water-soluble near-infrared (NIR) fluorescent labeling probes, named KSQ-3 and -4, which are based on a squaraine backbone, were synthesized and applied to biological labeling. The presented results demonstrate that the large, planar and hydrophobic squaraine dye becomes fully soluble in aqueous solution by the introduction of several sulfo group terminated alkyl substituents. Especially KSQ-4, which is substituted with four sulfo groups, exhibited perfect water solubility and significant fluorescence emission at the NIR region (817 nm) in the presence of bovine serum albumin (BSA). BSA was covalently labeled with KSQ-4, and the conjugate showed a strong absorption peak at 787 nm, which indicates compatibility with commercially available NIR laser diodes used for exciting the fluorophore. Furthermore, strong fluorescence emission was observed at 812 nm (phi = 0.08).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cattle
  • Cyclobutanes / chemical synthesis
  • Cyclobutanes / chemistry*
  • Fluorescent Dyes / chemistry*
  • Heterocyclic Compounds, 3-Ring / chemical synthesis
  • Heterocyclic Compounds, 3-Ring / chemistry*
  • Infrared Rays
  • Molecular Structure
  • Phenols / chemical synthesis
  • Phenols / chemistry*
  • Sensitivity and Specificity
  • Serum Albumin, Bovine / analysis
  • Serum Albumin, Bovine / chemistry*
  • Solubility
  • Spectrometry, Fluorescence / methods
  • Staining and Labeling
  • Water / chemistry


  • Cyclobutanes
  • Fluorescent Dyes
  • Heterocyclic Compounds, 3-Ring
  • KSQ-3
  • KSQ-4
  • Phenols
  • squaraine
  • Water
  • Serum Albumin, Bovine