possibly ultimately in vivo) bio-imaging applications. One of the technological
objectives of this study will be to bridge the gap between new cell biomarkers and
two-photon NLO-phores design.
Designing highly efficient second-order and third-order nonlinear optical
(NLO) chromophores is largely a matter of finely combining a high density of
delocalized electrons in a symmetrical or unsymmetrical environment. For
enhancing NLO efficiencies, innovative strategies should be explored in the future.
To enhance second-order v
(2) efficiencies, metal core-doping is a promising strategy
as it distorts the metallic atom core [62]. Also, enhanced fluorescence of AuNCs by
silver doping was reported by Guevel et al. [63] and Brach et al. [64]. Therefore, to
enhance third-order v
(3) efficiencies, silver doping can be coupled to ligand shell
rigidity as the latter favors the radiative properties of NCs. This route was recently
explored by Brach et al. [64] who showed that AuAgNCs exhibit two-photon
excited luminescence (2PL) emission and second-harmonic generation (SHG) and
that these properties remain the same in the liquid crystalline matrix. An alternative
route to enhance third-order v
(3) efficiencies is to use the concept of “rigidity.” We
recently pushed forward this strategy consisting in using bulky counter-ions to
enhance the luminescence in the nonlinear optical regime. We showed that by an
appropriate choice of bulky counter-ions and of solvent, a 30-fold increase in TPEF
signal in the red for glutathione-protected gold clusters could be obtained. A simple
way to increase the rigidity of the protective shell is to replace thiolated ligands by
proteins instead. Bovine serum albumin-Au 25 NCs indeed exhibit an efficient
two-photon absorption followed by blue [65] to red to near-infrared photoluminescence [66]. Using (bio)organic thiolate templates, further enhancement can be
achieved by increasing the rigidity of the metal–sulfur interface. The precise tailoring of the hydrophilicity/hydrophobicity balance on the gold or silver NC surfaces can be a way to increase the fluorescence signal [67, 68].
Acknowledgements Most of the work presented would not have been possible without the
fruitful collaborations with Isabelle Russier-Antoine, Franck Bertorelle, Željka Sanader, Marjan
Krstić, Philippe Dugourd, Pierre-François Brevet, and Vlasta Bonačić-Koutecký. Therefore, I wish
to express my deepest gratitude to them. Furthermore, I would like to acknowledge financial
support of the French-Croatian project “International Laboratory for Nano Clusters and Biological
Aging, LIA NCBA.”
References
1. de Meulenaere E, Nguyen Bich N, de Wergifosse M, van Hecke K, van Meervelt L,
Vanderleyden J, Champagne B, Clays K (2013) Improving the second-order nonlinear optical
response of fluorescent proteins: the symmetry argument. J Am Chem Soc 135:4061–4069
2. Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M (2008) Enhanced
two-photon absorption of organic chromophores: theoretical and experimental assessments.
Adv Mater 20:4641–4678
156
R. Antoine
objectives of this study will be to bridge the gap between new cell biomarkers and
two-photon NLO-phores design.
Designing highly efficient second-order and third-order nonlinear optical
(NLO) chromophores is largely a matter of finely combining a high density of
delocalized electrons in a symmetrical or unsymmetrical environment. For
enhancing NLO efficiencies, innovative strategies should be explored in the future.
To enhance second-order v
(2) efficiencies, metal core-doping is a promising strategy
as it distorts the metallic atom core [62]. Also, enhanced fluorescence of AuNCs by
silver doping was reported by Guevel et al. [63] and Brach et al. [64]. Therefore, to
enhance third-order v
(3) efficiencies, silver doping can be coupled to ligand shell
rigidity as the latter favors the radiative properties of NCs. This route was recently
explored by Brach et al. [64] who showed that AuAgNCs exhibit two-photon
excited luminescence (2PL) emission and second-harmonic generation (SHG) and
that these properties remain the same in the liquid crystalline matrix. An alternative
route to enhance third-order v
(3) efficiencies is to use the concept of “rigidity.” We
recently pushed forward this strategy consisting in using bulky counter-ions to
enhance the luminescence in the nonlinear optical regime. We showed that by an
appropriate choice of bulky counter-ions and of solvent, a 30-fold increase in TPEF
signal in the red for glutathione-protected gold clusters could be obtained. A simple
way to increase the rigidity of the protective shell is to replace thiolated ligands by
proteins instead. Bovine serum albumin-Au 25 NCs indeed exhibit an efficient
two-photon absorption followed by blue [65] to red to near-infrared photoluminescence [66]. Using (bio)organic thiolate templates, further enhancement can be
achieved by increasing the rigidity of the metal–sulfur interface. The precise tailoring of the hydrophilicity/hydrophobicity balance on the gold or silver NC surfaces can be a way to increase the fluorescence signal [67, 68].
Acknowledgements Most of the work presented would not have been possible without the
fruitful collaborations with Isabelle Russier-Antoine, Franck Bertorelle, Željka Sanader, Marjan
Krstić, Philippe Dugourd, Pierre-François Brevet, and Vlasta Bonačić-Koutecký. Therefore, I wish
to express my deepest gratitude to them. Furthermore, I would like to acknowledge financial
support of the French-Croatian project “International Laboratory for Nano Clusters and Biological
Aging, LIA NCBA.”
References
1. de Meulenaere E, Nguyen Bich N, de Wergifosse M, van Hecke K, van Meervelt L,
Vanderleyden J, Champagne B, Clays K (2013) Improving the second-order nonlinear optical
response of fluorescent proteins: the symmetry argument. J Am Chem Soc 135:4061–4069
2. Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M (2008) Enhanced
two-photon absorption of organic chromophores: theoretical and experimental assessments.
Adv Mater 20:4641–4678
156
R. Antoine