A superior bright NIR luminescent nanoparticle preparation and indicating calcium signaling detection in cells and small animals

Jian Zhang,Joseph R Lakowicz

doi:10.1186/s13578-018-0235-1

Jian Zhang, Joseph R Lakowicz

Open Access

https://doi.org/10.1186/s13578-018-0235-1

Copy DOI

Abstract

BackgroundNear-field fluorescence (NFF) effects were employed to develop a novel near-infrared (NIR) luminescent nanoparticle (LNP) with superior brightness. The LNP is used as imaging contrast agent for cellular and small animal imaging and furthermore suggested to use for detecting voltage-sensitive calcium in living cells and animals with high sensitivity.ResultsNIR Indocyanine green (ICG) dye was conjugated with human serum albumin (HSA) followed by covalently binding to gold nanorod (AuNR). The AuNR displayed dual plasmons from transverse and longitudinal axis, and the longitudinal plasmon was localized at the NIR region which could efficiently couple with the excitation and emission of ICG dye leading to a largely enhanced NFF. The enhancement factor was measured to be about 16-fold using both ensemble and single nanoparticle spectral methods. As an imaging contrast agent, the ICG–HSA-Au complex (abbreviate as ICG-Au) was conjugated on HeLa cells and fluorescence cell images were recorded on a time-resolved confocal microscope. The emission signals of ICG-Au complexes were distinctly resolved as the individual spots that were observed over the cellular backgrounds due to their strong brightness as well as shortened lifetime. The LNPs were also tested to have a low cytotoxicity. The ICG-Au complexes were injected below the skin surface of mouse showing emission spots 5-fold brighter than those from the same amount of free ICG–HSA conjugates.ConclusionsBased on the observations in this research, the excitation and emission of NIR ICG dyes were found to be able to sufficiently couple with the longitudinal plasmon of AuNRs leading to a largely enhanced NFF. Using the LNP with super-brightness as a contrast agent, the ICG-Au complex could be resolved from the background in the cell and small animal imaging. The novel NIR LNP has also a great potential for detection of voltage-gated calcium concentration in the cell and living animal with a high sensitivity.

Highlights

Near-field fluorescence (NFF) effects were employed to develop a novel near-infrared (NIR) luminescent nanoparticle (LNP) with superior brightness
The Indocyanine green (ICG) dyes were first conjugated with the human serum albumin (HSA) molecules to form the ICG–HSA conjugates, and the conjugates were covalently bound onto the surfaces of gold nanorod (AuNR)
Too many ICG dyes on one HSA molecule would result in self-quenching among the fluorophores and too few dyes in one HSA molecule would result in a low brightness

Summary

Introduction

Near-field fluorescence (NFF) effects were employed to develop a novel near-infrared (NIR) luminescent nanoparticle (LNP) with superior brightness. The LNP is used as imaging contrast agent for cellular and small animal imaging and suggested to use for detecting voltage-sensitive calcium in living cells and animals with high sensitivity. With a change of concentration or environment of Ca2+ ions in the cell, the fluorescence signal from the indicator is altered. This method can be used for exploring the intracellular calcium concentration and gradient of calcium ion at the cellular level as well as in the living animals [7, 8]. The monitoring voltage-gated imaging calcium has become an important topic in the calcium channel detection because the calcium signals exert their highly specific functions in the well-defined cells or/and small animals

Methods

Results

Discussion

Conclusion