Thionine Molecules Research Articles

Molecular modification enables CO2 electroreduction to methane on platinum surface in acidic media

Abstract Cu-based materials can produce hydrocarbons in CO2 electroreduction (CO2RR), but their stability is still needed to be enhanced particularly in acidic media. Metallic Pt are highly stable in both acidic and alkaline media, yet they are rarely utilized in CO2RR, due to the competitive activity in hydrogen evolution reaction (HER). In this research, abundant thionine (Th) molecules are stably confined within Pt nanocrystals via a molecularly doping strategy. Pt surface is successfully modulated by these Th molecules, and thereby the dominant HER activity is converted to CO2RR activity. CO2 could be electroreduced to CH4 using organic molecular modified Pt-based catalysts for the first time. Specifically, this composite catalyst maintains more than 100-hour stability in strong acid conditions (pH 1), even comparable to those state-of-the-art CO2RR catalysts. In-situ spectroscopic analysis and theoretical calculations reveal that the molecular modification can decrease the energy barrier for *COOH formation, and guarantee the sufficient local *H near Pt surface. Additionally, the *H derived from H2O dissociation is favorable for the *CO hydrogenation pathway towards *CHO, eventually leading to the formation of CH4. This strategy might be easily applied to microenvironment and interface regulation in other electrocatalytic reactions.

Electrochemical biosensor based on Au5Ir@graphene quantum dot nanocomposite and DNA walker for detection of atrazine in environmental water with extremely high sensitivity and selectivity

Existing electrochemical sensors are inadequate for detecting pesticides at extremely low level. This study presents an electrochemical biosensor based on gold-iridium alloy (Au5Ir) and DNA walker for detection of atrazine. Au5Ir@RFBP-GQD nanocomposite was synthesized via the reduction of chloroauric acid and iridium trichloride using arginine and folic acid-functionalized boron and phosphorus-doped graphene quantum dot (RFBP-GQD). The synergy between Au and Ir, along with the formation of Schottky heterojunction, enhances the catalytic activity. The Au5Ir was covalently conjugated with hairpin DNA and thionine forming a redox probe that was used to construct the electrochemical biosensor for atrazine detection, coupled with a DNA walker mechanism. Atrazine triggers the DNA walker, leading to the introduction of multiple redox probes onto the gold electrode surface. The oxidation and reduction of thionine molecules within these redox probes elicit highly sensitive electrochemical response. The integration of Au5Ir@RFBP-GQD catalysis with the DNA walker results in significant signal amplification. The biosensor exhibits superior sensitivity and selectivity compared to other atrazine sensors, with a linear detection range from 1×10−18 to 1×10−12 M and a low detection limit of 3.4×10−19 M (S/N=3). The proposed analytical method was successfully used for detection of atrazine in environmental water.

An electrochemical biosensor for T4 polynucleotide kinase activity assay based on host-guest recognition between phosphate pillar[5]arene@MWCNTs and thionine.

T4 polynucleotide kinase helps with DNA recombination and repair. In this study, an electrochemical biosensor was developed for a T4 polynucleotide kinase activity assay and inhibitor screening based on phosphate pillar[5]arene and multi-walled carbon nanotube nanocomposites. The water-soluble pillar[5]arene was employed as the host to complex thionine guest molecules. The substrate DNA with a 5'-hydroxyl group initially self-assembled on the gold electrode surface through chemical adsorption of the thiol group, which was phosphorylated in the presence of T4 polynucleotide kinase. Titanium dioxide nanoparticles served as a bridge to link phosphorylated DNA and phosphate pillar[5]arene and multi-walled carbon nanotube composite due to strong phosphate-Ti4+-phosphate chemistry. Through supramolecular host-guest recognition, thionine molecules were able to penetrate the pillar[5]arene cavity, resulting in an enhanced electrochemical response signal. The electrochemical signal is proportional to the T4 polynucleotide kinase concentration in the range of 10-5 to 15 U mL-1 with a detection limit of 5 × 10-6 U mL-1. It was also effective in measuring HeLa cell lysate-related T4 polynucleotide kinase activity and inhibitor screening. The proposed method offers a unique sensing platform for kinase activity measurement, holding great potential in nucleotide kinase-target drug development, clinical diagnostics, and inhibitor screening.

Achieving enhanced sensitivity and accuracy in carcinoembryonic antigen (CEA) detection as an indicator of cancer monitoring using thionine/chitosan/graphene oxide nanocomposite-modified electrochemical immunosensor

The current study has focused on electrochemical immunosensing of carcinoembryonic antigen (CEA) employing an immobilized antibody on a thionine, chitosan, or graphene oxide nanocomposite modified glassy carbon electrode (anti-CEA/THi–CS–GO/GCE) as an indicator of cancer monitoring. THi–CS–GO nanocomposites were made using ultrasonication, and analyses of their morphology and crystal structure using SEM, FTIR, and XRD showed that thionine and chitosan molecules were intercalated with stacking interactions with both the top and bottom of GO nanosheets. Electrochemical experiments revealed anti-CEA, THi–CS–GO/GCE to have exceptional sensitivity and selectivity towards CEA compounds. The detection limit value was established to be 0.8 pg/mL when it was discovered that variations in the decrease peak current were directly proportional to the logarithm concentration of CEA over a wide range from 10-3 to 104 ng/mL. Results of testing the immunosensor's application capability for detecting CEA in a sample of human serum show that ELISA and DPV results are very congruent. The produced immunosensor demonstrated adequate immunosensor precision in determining CEA in prepared genuine samples of human serum and clinical applications.

Potassium cobalt hexacyanoferrate as a peroxidase mimic for electrochemical immunosensing of Lactobacillus rhamnosus GG

In this paper, the potassium cobalt hexacyanoferrate (II), K2CoFe(CN)6, with peroxidase-like activity was used for the fabrication of a novel label-free Lactobacillus rhamnosus GG (LGG) electrochemical immunosensor. The K2CoFe(CN)6 nanocubes were made by a simple hydrothermal method and followed by low-temperature calcination. In addition to structural characterization, the peroxidase-mimicking catalytic property of the material was confirmed by a chromogenic reaction. It is known that H2O2 can oxidize electroactive thionine molecules under the catalysis of horseradish peroxidase (HRP). In this nanozyme-based electrochemical immunoassay, due to the steric hindrance, the formation of immune-complex of LGG and LGG antibody on the modified GCE inhibits the catalytic activity of the peroxidase mimics of K2CoFe(CN)6 and thus reduced the current signal. Therefore, the developed electrochemical immunosensor achieved quantitative detection of LGG. Under optimal conditions, the linear range of the sensor was obtained from 101 to 106 CFU mL−1 with a minimum detection limit (LOD) of 12 CFU mL−1. Furthermore, the immunosensor was successfully applied in the quantitative detection of LGG in dairy product samples with recoveries ranging from 93.2% to 106.8%. This protocol presents a novel immunoassay method, which provides an alternative implementation pathway for the quantitative detection of microorganisms.

Label-free electrochemical immunosensor for highly sensitive COVID-19 spike protein detection

The ongoing coronavirus pandemic responsible for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly increased the rate of global death and infections due to variant mutations (such as Delta and Omicron). While specifically developed and approved vaccines can limit the spread of disease in a population and severity of resulting symptoms, none have been demonstrated to effectively prevent infection altogether. Thus, reliable early diagnosis of COVID-19 is critical to identify positive cases to help contain the outbreak. Herein we report a label-free electrochemical immunosensor for rapid diagnosis of COVID-19 by using nitrogen-doped holey graphene (N-HRGO) as a nanocarrier decorated with thionine (TH) molecules as electrochemical indicators. With the spike protein located on the surface of the COVID-19 particles as the model target, the as-prepared electrochemical immunosensor could detect the presence of the COVID-19 spike protein over a wide linear range (1 pg mL−1−10 ng mL−1) with a low detection limit (0.3 pg mL−1). In addition, the developed electrochemical immunosensor exhibited an excellent selectivity (with insignificant current changes towards interfering proteins comparing with COVID-19 spike protein), a good reproducibility and long-term storage stability. Importantly, the electrochemical immunosensor thus developed could successfully and reliably detect the spike protein of COVID-19 in saliva and human serum complex samples. Thus, the as-prepared label-free electrochemical immunosensor can achieve rapid and sensitive detection of the COVID-19 spike protein, as a promising clinical diagnosis tool in monitoring the progression of COVID-19.

Ultrasensitive electrochemical detection of hepatitis b virus surface antigen based on hybrid nanomaterials

This study investigates a sandwich-type electrochemical immunosensor for the determination of Hepatitis B Virus Surface Antigen (HBsAg) as the biological target. Capture antibody (Ab1) was immobilized on the synthesized magnetic nanoparticles coated with silica and APTES (MNP@SiO2@APTES). A reporter antibody (Ab2) was assembled on gold nanoparticle-Thionine (GNP), which made use of Th as a redox probe for electrochemical detection. Each gold nanoparticle holds a lot of thionine molecules on its surface. Therefore, after the formation of a sandwich-type probe a magnified electrical signal is achieved. On the other hand, a magnet has been implemented in the electrode. Thus, Magnetic nanoparticle (MNP) acts as the probe collector and fixator component on the electrode's surface, assuring a strong and prolonged interaction of the ultimate sandwich-type probe with the electrode. Under optimal conditions, the current signal increased with the increment of HBsAg concentration in the linear dynamic range of 0.001 fg/ml-2 ng/ml with a detection limit (LOD) of 0.3ag/ml (S/N = 3). Furthermore, the assembled immunosensor successfully quantified the HBsAg concentration in serum. The successful results imply the high potential of the proposed immunosensor in developing ultrasensitive and rapid diagnostic platforms.

New immunoprobe: Dual-labeling ZIF-8 embellished with multifunctional bovine serum albumin lamella for electrochemical immunoassay of tumor marker

A new immunoprobe, which can initiate the sedimentation of Ag nanoparticles (NPs) on an electrode surface, was developed for the electrochemical detection of carbohydrate antigen 72–4 (CA 72–4). To design the immunoprobe, zeolitic imidazolate frameworks (ZIFs) were employed as the carrier to enrich thionine molecules, then bovine serum albumin (BSA) was modified on the electrode surface. Advantageously, BSA, served as an anchor to further attach the labeling antibodies (Ab2) and alkaline phosphatase (ALP) to also be modified on the surface through covalent bonding. To construct the immunosensor, multiwalled carbon nanotube-graphene oxide composites were employed to provide active sites, and the electrodeposited Au NPs were used to immobilize coating antibodies. In the presence of CA 72–4, a sandwich immunosensor was established, and a cascade reaction was initiated to deposit Ag NPs under the catalysis, which can further improve the conductivity of electrode interface. Under the optimal conditions, the immunosensor displayed excellent performance with a wide linear range from 1 μU mL−1 to 10 U mL−1 and an ultralow detection limit of 0.438 μU mL−1 (S/N = 3).

Thionine-doped nanometer-sized silica conjugated with phenylboronic acid: An innovative recognition/signal element for voltammetric aptasensing of colorectal cancer-related carcinoembryonic antigen

This work reports the proof-of-concept of novel and powerful enzyme-free voltammetric aptasensor of colorectal cancer-related carcinoembryonic antigen (CEA) without the participation of any antibodies. The aptasensor was prepared by means of immobilizing thiolated CEA aptasensor on gold nanoparticle-deposited glassy carbon electrode. Thionine-doped silica nanoparticles were synthesized via reverse micelle method and functionalized with p-aminophenylboronic acid by the epoxy-amino reaction for specific conjugation of subsequent CEA glycoprotein. In the presence of target CEA, the analyte initially reacted with immobilized aptamer on the electrode to generate an aptamer-CEA complex. Then, phenylboronic acid immobilized on silica nanoparticle specifically recognized with CEA glycoprotein based on sugar-boronic acid interaction to form a sandwich-type complex on the electrode. Within the applied potentials, the doped thionine molecules into the silica nanoparticles were used as the electron mediators to produce well-defined voltammetric signals (vs. Ag/AgCl). Under optimum conditions, the voltammetric peak current increased linearly within the CEA concentration in the range from 1.0 pg mL−1 to 10 ng mL−1, and the limit of detection was 0.49 pg mL−1 at 3δ. A repeatability and intermediate precision of ≤8.7% was accomplished at CEA levels in the batch-to-batch mode. The method was highly specific for CEA over other disease-related biomarkers. The accuracy and inter-laboratory validation of this system were evaluated for target CEA detection in human serum specimens, and the results were in accordance with those obtained by a commercial ELISA.

Ultrasensitive Electrochemical DNA Biosensor Fabrication by Coupling an Integral Multifunctional Zirconia-Reduced Graphene Oxide-Thionine Nanocomposite and Exonuclease I-Assisted Cleavage.

In this work, a simple but sensitive electrochemical DNA biosensor for nucleic acid detection was developed by taking advantage of exonuclease (Exo) I-assisted cleavage for background reduction and zirconia-reduced graphene oxide-thionine (ZrO2-rGO-Thi) nanocomposite for integral DNA recognition, signal amplification, and reporting. The ZrO2-rGO nanocomposite was obtained by a one-step hydrothermal synthesis method. Then, thionine was adsorbed onto the rGO surface, via π-π stacking, as an excellent electrochemical probe. The biosensor fabrication is very simple, with probe DNA immobilization and hybridization recognition with the target nucleic acid. Then, the ZrO2-rGO-Thi nanocomposite was captured onto an electrode via the multicoordinative interaction of ZrO2 with the phosphate group on the DNA skeleton. The adsorbed abundant thionine molecules onto the ZrO2-rGO nanocomposite facilitated an amplified electrochemical response related with the target DNA. Since upon the interaction of the ZrO2-rGO-Thi nanocomposite with the probe DNA an immobilized electrode may also occur, an Exo I-assisted cleavage was combined to remove the unhybridized probe DNA for background reduction. With the current proposed strategy, the target DNA related with P53 gene could be sensitively assayed, with a wide linear detection range from 100 fM to 10 nM and an attractive low detection limit of 24 fM. Also, the developed DNA biosensor could differentiate the mismatched targets from complementary target DNA. Therefore, it offers a simple but effective biosensor fabrication strategy and is anticipated to show potential for applications in bioanalysis and medical diagnosis.

Double molecular recognition strategy based on boronic acid-diol and NHS ester-amine for selective electrochemical detection of cerebral dopamine.

Electrochemical detection of dopamine (DA) usually depends on electrochemical oxidation of DA. This conventional method can hardly provide sufficient sensitivity and selectivity in the determination of the cerebral DA down to nanomolar level, because the ability of DA to be electrochemically oxidized is limited and many electroactive interferents are also oxidized at a similar potential with DA. Here, an electrochemical assay based on a double molecular recognition strategy has been proposed and proved to be of high sensitivity and selectivity for DA measurement in the cerebral system. 3,3'-Dithiodipropionic acid di(N-hydroxysuccinimide ester) (DSP) was anchored on the electrode surface to capture DA target through the specific reaction between N-hydroxysuccinimide (NHS) ester and amine. The captured DA endowed the electrode with a layer of diol groups, which further reacted with the boronic acid to trap of 4-mercaptophenylboronic acid (MBA) molecules, thus leading to the conjunction of electroactive thionine (Th) molecules on the electrode for signal readout. In addition, an Au nanostructure was employed to enhance signal amplification and facilitate the double molecular recognition process. As a consequence, this method was able to quantify DA from 1 to 300nM with a detection limit of 0.74nM, which exhibited a high specificity against cerebral interferents. Furthermore, the practicability of this platform was successfully demonstrated through determination of the dynamics of cerebral DA in the events of high K+ and nomifensine retromicrodialysis. Graphical abstract.

Design and Application of an Optical pH Sensor Based on Thionine Doped Modified Sol–Gel Film

An optical chemical sensor was constructed by incorporating the thionine (TN) dye molecules into a sol–gel silica matrix and it was used for determination the pH of the aqueous solutions. Entrapment of TN dye into the silica matrix was made via an acid-catalyzed sol–gel reaction of tetraethyl orthosilicate (TEOS) and the thionine molecules. The thionine molecules in the immobilized form, showed a different spectral behavior than its solution form. After optimizing the experimental conditions, the fabricated optical sensor showed a fast response to pH changes in the range of 11–13 with a response time about one minute. Also, the obtained results indicate that the proposed pH sensor, has a good reproducibility, reversibility, stability and low leaching and a long lifetime of over 7 months.

A corrosion study: Use of Thionine dye having structurally metachromatic influence

In the present study, it was examined the inhibition performance of Thionine, which has metachromatic properties and is generally used as a vital dye for different staining of nucleus and cytoplasm, on mild steel in acidic medium as a different application area which is not existed in the literature. In order to understand how Thionine interacts with the mild steel surface, different types of adsorption isotherms were plotted and it was seen obeyed the Langmuir isotherm. The test results revealed that as the inhibitor concentration increased at each temperature, the corrosion current density (icorr) values diminished and accordingly, the inhibition efficiencies (η%) increased slightly. The results indicated that the Thionine molecules continued to be adsorbed onto the metal surface to some extent, even when elevated to high temperatures. Thermodynamic adsorption parameters revealed a strong and chemical interaction between Thionine and mild steel. It was determined that the Thionine acted as a mixed-type inhibitor on the mild steel surface. Finally, field emission scanning electron (FESEM) and atomic force microscopy (AFM) analyses were performed to determine the surface characteristics.

Ultrasensitive and robust electrochemical sensing platform for the detection of squamous cell carcinoma antigen using water-soluble pillar [5]arene-Pd/MoS2 nanocomposites

Designing and developing the electrochemical analytical equipment based on biomimetic materials and supramolecular chemistry has gained wide attention in recent years because the incorporation of molecular recognition and nanomaterials can effectively enhance the analytical performance and robustness of electrochemical/bio sensors. Herein, an ultrasensitive and robust sandwich-type electrochemical immunosensor has been fabricated for quantitative determination of squamous cell carcinoma antigen (SCCA), a well-defined biomarker of squamous cell carcinoma in human serum. The water-soluble pillar[5]arene functionalized Pd/MoS2 nanosheets are prepared to immobilize the electroactive thionine molecules through host-guest interaction, and employed as labels to capture detection antibodies for signal amplification. Due to the outstanding electrochemical properties, large specific surface area, and excellent catalytic activity of Thi@WP5-Pd/MoS2 nanocomposites, the robust current response towards the oxidation of thionine are achieved. Besides, the Au nanoparticles decorated TiO2 nanosheets are applied to primary antibodies binding via Au–N bond. Under the optimized conditions, the prepared immunosensor offers a wide linear range from 0.001 to 10 ng mL−1 with a low detection limt of 0.14 pg mL−1. The satisfying results of analyzing SCCA in human serum suggest a promising application in the clinical monitoring.

Control of direction of nonradiative resonance energy transfer in hybrid associates of colloidal Ag2S/TGA QDs with thionine molecules

The possibilities of changing the direction of electronic excitations transfer in hybrid associates are analyzed. Studies of hybrid associates of colloidal Ag2S quantum dots (QDs), stabilized with thioglycolic acid (TGA) with thionine (TH+) molecules, were realized by transmission electron microscopy, absorption and luminescence spectroscopy, and photoluminescence decay studies (time correlated single photon counting). The increase in the average size of QDs from 1.8 to 5.5 nm, as well as shift of QDs luminescence peak from 630 to 950 nm were observed, when TGA molecules together with sodium sulfide was used as a sulfur precursor. The hybrid association of QDs (1.8 nm) with TH+ molecules leads to quenching of QDs luminescence in the luminescence band with peak at 630 nm and simultaneously decreases in luminescence lifetime from 13.7 to 6.5 ns. In the case of QDs association with a luminescence peak at 950 nm with TH+ molecules, it leads to quenching of TH+ luminescence and decrease in its luminescence lifetime from 0.43 to 0.3 ns. It is concluded that the decreasing in the luminescence lifetime is caused by nonradiative resonance energy transfer between components of associates. An increase in the average size of QDs leads to a change in the direction of energy transfer between associate components.

Resonant Nonradiative Energy Transfer in Hybrid Associates of Thionine Molecules and Ag2S Colloidal Quantum Dots with Different Luminescence Mechanisms

Resonant Nonradiative Energy Transfer in Hybrid Associates of Thionine Molecules and Ag2S Colloidal Quantum Dots with Different Luminescence Mechanisms

Luminescence and Nonlinear Optical Properties of Hybrid Associates of Ag2S Quantum Dots with Molecules of Thiazine Dyes

Spectral, kinetic, and nonlinear optical regularities that demonstrate the exchange of electronic excitations between the components of hybrid associates of Ag2S colloid quantum dots (1.7–1.8 nm) in gelatin with molecules of thiazine dyes (Ds) are found. When the IR luminescence of Ag2S quantum dots (QDs) is excited by radiation from the thionine absorption region, its enhancement due to nonradiative resonant energy transfer is observed. The association with methylene-blue molecules blocked the IR luminescence of Ag2S QDs upon its excitation by radiation from the absorption region of the dye due to the transfer of charge carriers. It is demonstrated that the hybrid association of thionine molecules and Ag2S QDs adversely affects the nonlinear optical properties of the latter, which manifests itself in inverse saturated absorption by the action of 10-ns second-harmonic pulses (532 nm) of a Nd3+:YAG laser. For the associates of Ag2S QDs with methylene-blue molecules, the radiation focusing caused by the transfer of charge carriers from the dye and the change in the population of small traps in nanocrystals is found. It is concluded that the direction of the transfer of electronic excitations and the photophysical processes in these objects are determined by the mutual arrangement of the HOMO–LUMO levels of the dye with respect to the levels of dimensional quantization of the Ag2S QDs.

Primary Photoprocesses in Thionine in Supramolecular Complexes with Cucurbit[7,8]urils in Water

Thionine (ThH+) molecules form monomeric ThH+@CB7 (1: 1) and dimeric 2ThH+@CB8 (2: 1) complexes with cucurbit[7,8]urils (CB7) and (CB8) in water. Unlike the case free ThH+ molecules, the absorption spectrum of the complexes is characterized by a hypsochromic shift of the maximum by 6 and 41 nm for ThH+@CB7 and 2ThH+@CB8, respectively. The ThH+@CB7 complexes exhibit fluorescence, unlike the nonfluorescing 2ThH+@CB8 complexes. The monomeric complexes undergo intersystem crossing to the triplet state with a lifetime of 14 μs. The dimeric complexes have a very low quantum yield of the triplet state. The triplet state of the dimeric complexes was populated by photosensitized excitation by triplet–triplet energy transfer. The lifetime of the triplet state is ≈50 μs.

Electrochemical integrated paper-based immunosensor modified with multi-walled carbon nanotubes nanocomposites for point-of-care testing of 17β-estradiol

17β-estradiol (17β-E2) plays a critical role in regulating reproduction in human, there is therefore an urgent need to detect it sensitively and precisely in a cost-effective and easy method. In this paper, a label-free integrated microfluidic paper-based analytical device was developed for highly sensitive electrochemical detection of 17β-E2. The microfluidic channel of the paper-based sensor was fabricated with wax printing and the three electrodes, including working, counter and reference electrode were screen-printed. Multi-walled carbon nanotubes (MWCNTs)/ thionine (THI)/ gold nanoparticles (AuNPs) Nano composites were synthesized and coated on screen-printed working electrode (SPWE) for the immobilization of anti-E2. In this electro-chemical system of paper-based immunoassay, THI molecules serving as an electrochemical mediator while MWCNTs and AuNPs, due to their excellent electrical conductivities, could accelerate electron transfer for the signal amplification. Experimental results revealed that the immunoassay is able to detect 17β-E2 as low as 10 pg mL−1, with a linear range from 0.01 to 100 ng mL−1. This microfluidic paper-based immunosensor would provide a new platform for low cost, sensitive, specific, and point-of-care diagnosis of 17β-E2.

Control of the direction of energy transfer in associates of colloidal quantum dots Ag2S/TGA and dye molecules

The possibilities for changing the direction of energy transfer in hybrid associates of colloidal Ag2S quantum dots (QDs) and thionine molecules are analyzed. The studies were performed by transmission electron microscopy, absorption and luminescence spectroscopy, PL decay study (time correlated single photon counting). An increasing of the average QDs size from 1.8 nm to 5.5 nm, and also a shift of the luminescence band from 630 nm to 950 nm, were found at using of TGA molecules and sodium sulfide as a sulfur precursor. Hybrid association of QDs (1.8 nm) with TH+ molecules leads to quenching of QDs luminescence with a simultaneous reduction of the luminescence lifetime from 13.7 to 6.5 ns. An association of QDs with a luminescence band maximum of 950 nm with TH+ molecules leads to quenching of TH+ luminescence and a reduction in its lifetime of luminescence from 0.43 to 0.3 ns. It was concluded that the reduction of lifetime of luminescence caused by the resonant nonradiative energy transfer between the components of the associates. An increasing in the average size of QDs leads to a change in the direction of energy transfer between the components of the associates.