# Quantum Dot Conjugates in Functional Imaging and Highly Sensitive Biochemical Assays

I Nabiev

► To cite this version:

I Nabiev. Quantum Dot Conjugates in Functional Imaging and Highly Sensitive Biochemical Assays. KnE Energy, 2018, 3 (2), pp.287-291. 10.18502/ken.v3i2.1824. hal-03028883

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Distributed under a Creative Commons Attribution 4.0 International License
Conference Paper

# Quantum Dot Conjugates in Functional Imaging and Highly Sensitive Biochemical Assays

Nabiev I.R.<fnref n="1" />,<fnref n="2" />

## Abstract

Semiconductor quantum dots (QDs) are characterized by orders of magnitude higher multiphoton linear absorption cross-sections compared with conventional organic dyes. Combined with the QD photoluminescence quantum yield approaching 100%, this fact opens new prospects for the two-photon functional imaging of QDs tagged with highly specific recognition molecules, thus permitting high-quality images with a very low autofluorescence contribution to be obtained. Additionally, unique photostability of QDs enables signal accumulation and significant enhancement of the sensitivity of routine biochemical and immunohistochemical assays to be obtained when the conjugates of QDs, instead of organic dyes, are used.

**Keywords:** Nanocrystals, semiconductor quantum dots, FRET, single-domain antibodies, imaging, multiphoton.

## 1. Introduction

The existing photonic techniques of *in vitro* and *in vivo* diagnostics and imaging are mainly limited by the difficulties related to dye photobleaching and their detection in the optically noisy cell and tissue environment [1-3]. Semiconductor quantum dots (QDs) have emerged as alternative tools for cellular labelling, biochemical sensing, probing biocatalysed reactions, and drug delivery [4-7].

## 2. Materials and Methods

CdSe/ZnS QDs were synthesized as described earlier [6]. Briefly, two solutions were prepared, one containing 10 g of trioctylphosphine oxide (TOPO, Aldrich) and 5 g of hexadecylamine (HDA, Fluka), and the other containing 80 mg of elemental Se and 110

Corresponding Author:
Nabiev I.R.
igor.nabiev@univ-reims.fr

Received: 17 January 2018

Accepted: 25 March 2018

Published: 17 April 2018

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© Nabiev I.R.. This article is distributed under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use and redistribution provided that the original author and source are credited.

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<footnote n="1">¹Laboratoire de Recherche en Nanosciences, EA4682-LRN, Université de Reims Champagne-Ardenne, Reims, France</footnote>

<footnote n="2">²National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, Moscow, 115409, Russia</footnote>
µl of dimethylcadmium (Strem, 97%) in 1 ml of trioctylphosphine (TOP, Fluka). The first solution was dried, degassed under vacuum at 180°C, purged with argon, and heated to 340°C under argon flow. Then, fast (<1 s) injection of the second solution into the first one yielded CdSe cores approximately 2 nm in size. Further growth of CdSe cores to the desired size (and, hence, the desired fluorescence colour) was induced by prolonged refluxing of the solution at 280°C. After completion of the process, CdSe cores were precipitated at 60°C with methanol, washed twice with methanol, and dried. In order to grow an epitaxial ZnS shell on the CdSe core, a powder of CdSe cores was dissolved in a mixture of 10 g of TOPO and 5 g of HDA. Once again, the mixture was dried and degassed under vacuum at 180°C and purged with argon. A solution containing 210 µl of hexamethyldisilthiane (Fluka) and 130 µl of diethylzinc (Strem, 97%) in 2 ml of TOPO was added dropwise to this mixture at 220°C under argon flow and intense stirring. The resultant colloidal solution of CdSe/ZnS nanoparticles was slowly cooled to 60°C, and the QDs precipitated with methanol were washed twice with methanol and dried. The synthesized QDs consisted of a CdSe core 4.0 nm in diameter and an epitaxial shell of several ZnS monolayers. The QDs were demonstrated to be highly homogeneous in size.

The QDs were solubilised in water as described earlier [6]. Briefly, QDs were first transferred to water after the attachment of DL-cysteine (Sigma) to their surface. The resultant water-soluble QDs displayed a bright orange photoluminescence (PL) with an emission maximum at 570 nm and a quantum yield close to 40% at room temperature. Then, DL-cysteine on the surface of the QDs was replaced with thiol-containing PEG derivatives with carboxyl or hydroxyl group at the end of the polymer chain or with a mixture of 10% of thiol-containing PEG derivatives with amino groups and 90% of thiol-containing PEG derivatives with hydroxyl groups. The samples were incubated overnight at +4°C, pre-cleaned by centrifugation with Amicon Ultra-15 filter units with a 10-kDa cut-off (Millipore), and finally purified from excess of ligands by gel exclusion chromatography on home-made Sephadex-25 (Sigma) columns.

The resultant water-soluble and stable QDs were conjugated with single-domain antibodies as described in Refs. [5, 6] or used in the form of a streptavidine conjugate, as described in Ref. [8]. The absorption and fluorescence spectra of the QDs were recorded using a Tecan Infinite 200 Pro microplate reader; the particle size and zeta potential of QDs were measured by dynamic light scattering using a Malvern Zetasizer Nano-ZS (Malvern Instruments Ltd., Worcestershire, UK).
## 3. Results and Discussion

As seen in Figure 1, the fluorescent signal from the assay employing strepta-QD obtained in the course of signal accumulation is more than two orders of magnitude stronger than the maximal signals that can be obtained in fluorogenic assays employing the best organic dyes under optimal conditions [8]. This shows that the use of QD-based labels in fluorogenic assays is decisively advantageous.

<figure><img src="image_3.png" /><figcaption>Figure 1: Comparative advantages of semiconductor quantum dots in *in vitro* assays.</figcaption></figure>

Analysis of variations of fluorescent signals as a result of their accumulation for typical *in vitro* biochemical fluorogenic assays employing two concentrations of the fluorescent streptavidin-QD800 conjugate and the most popular organic dyes. Adapted from Ref. [8].

Images of disseminated tumour cells identified by fluorescent microscopy with the single-domain antibody-quantum dot conjugate specific for the HER2 antigen are shown. Adapted from Ref. [10].

Moreover, QD conjugates with single-domain antibodies (sdAbs) have proved to be an efficient tool to detect disseminated human tumour cells and micrometastases by binding the HER2 receptor, an antigen overexpressed in metastatic breast tumour (Figure 2).

Additionally, the better diffusibility of sdAb-QD nanoprobes through tissues facilitates the access to small metastases and complex structures, making them powerful tools for cancer diagnostic and therapeutic applications [9].
<figure><img src="image_4.png" /><figcaption>**Figure 2:** Comparative advantages of semiconductor quantum dots in *in vivo* assays.</figcaption></figure>

## Acknowledgments

This study was supported by the Russian Science Foundation, contract no. 17-15-01533, and the Agence Nationale de Recherche (France) through the ICENAP project of the MERA.NET EU Programme (contract no. ANR-14-MERA-0002).

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