Near-infrared-emitting squaraine dyes with high 2PA cross-sections for multiphoton fluorescence imaging

ACS Appl Mater Interfaces. 2012 Jun 27;4(6):2847-54. doi: 10.1021/am300467w. Epub 2012 May 16.


Designed to achieve high two-photon absorptivity, new near-infrared (NIR) emitting squaraine dyes, (E)-2-(1-(2-(2-methoxyethoxy)ethyl)-5-(3,4,5-trimethoxystyryl)-1H-pyrrol-2-yl)-4-(1-(2-(2-methoxyethoxy)ethyl)-5-(3,4,5-trimethoxystyryl)-2H-pyrrolium-2-ylidene)-3-oxocyclobut-1-enolate (1) and (Z)-2-(4-(dibutylamino)-2-hydroxyphenyl)-4-(4-(dibutyliminio)-2-hydroxycyclohexa-2,5-dienylidene)-3-oxocyclobut-1-enolate (2), were synthesized and characterized. Their linear photophysical properties were investigated via UV-visible absorption spectroscopy and fluorescence spectroscopy in various solvents, while their nonlinear photophysical properties were investigated using a combination of two-photon induced fluorescence and open aperture z-scan methods. Squaraine 1 exhibited a high two-photon absorption (2PA) cross-section (δ2PA), ∼20 000 GM at 800 nm, and high photostability with the photochemical decomposition quantum yield one order of magnitude lower than Cy 5, a commercially available pentamethine cyanine NIR dye. The cytotoxicity of the squaraine dyes were evaluated in HCT 116 and COS 7 cell lines to assess the potential of these probes for biomedical imaging. The viability of both cell lines was maintained above 80% at dye concentrations up to 30 μM, indicating good biocompatibility of the probes. Finally, one-photon fluorescence microscopy (1PFM) and two-photon fluorescence microscopy (2PFM) imaging was accomplished after incubation of micelle-encapsulated squaraine probes with HCT 116 and COS 7 cells, demonstrating their potential in 2PFM bioimaging.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • COS Cells
  • Cell Survival / drug effects
  • Chlorocebus aethiops
  • Cyclobutanes / chemistry*
  • Cyclobutanes / toxicity
  • Fluorescent Dyes / chemistry*
  • Fluorescent Dyes / toxicity
  • HCT116 Cells
  • Humans
  • Micelles
  • Microscopy, Fluorescence
  • Phenols / chemistry*
  • Phenols / toxicity
  • Photons
  • Temperature
  • Water / chemistry


  • Cyclobutanes
  • Fluorescent Dyes
  • Micelles
  • Phenols
  • squaraine
  • Water