Salt-induced Aggregation Research Articles

Human papilloma virus DNA-biomarker analysis for cervical cancer: Signal enhancement by gold nanoparticle-coupled tetravalent streptavidin-biotin strategy

Human papillomavirus (HPV) is a double-standard DNA virus, as well as the source of infection to the mucous membrane. It is a sexually transmitted disease that brings the changes in the cervix cells. Oncogenes, E6 and E7 play a pivotal role in the HPV infection. Identifying these genes to detect HPV strains, especially a prevalent HPV16 strain, will bring a great impact. Among different sensing strategies for pathogens, the dielectric electrochemical biosensor shows the potential due to its higher sensitivity. In this research, HPV16-E7 DNA sequence was detected on the carbodiimidazole-modified interdigitated electrode (IDE) surface with the detection limit of 1 fM. To enhance the sensitivity, the target sequence was conjugated on gold nanoparticle (GNP) and attained detection to the level of 10 aM. This produced ~100 folds improvement in detecting HPV16-E7 gene and 4 folds increment in the current flow. The stability of HPV16-E7 DNA sequences on GNP was verified by the salt-induced GNP aggregation. The current system has shown the higher specificity by comparing against non-complementary and triple-mismatched DNA sequences of HPV16-E7. This demonstration in detecting HPV16-E7 using dielectric IDE sensing system with a higher sensitivity can be recommended for detecting a wide range of disease-causing DNA-markers.

Development of Label-Free Colorimetric Assay for MERS-CoV Using Gold Nanoparticles.

Worldwide outbreaks of infectious diseases necessitate the development of rapid and accurate diagnostic methods. Colorimetric assays are a representative tool to simply identify the target molecules in specimens through color changes of an indicator (e.g., nanosized metallic particle, and dye molecules). The detection method is used to confirm the presence of biomarkers visually and measure absorbance of the colored compounds at a specific wavelength. In this study, we propose a colorimetric assay based on an extended form of double-stranded DNA (dsDNA) self-assembly shielded gold nanoparticles (AuNPs) under positive electrolyte (e.g., 0.1 M MgCl2) for detection of Middle East respiratory syndrome coronavirus (MERS-CoV). This platform is able to verify the existence of viral molecules through a localized surface plasmon resonance (LSPR) shift and color changes of AuNPs in the UV–vis wavelength range. We designed a pair of thiol-modified probes at either the 5′ end or 3′ end to organize complementary base pairs with upstream of the E protein gene (upE) and open reading frames (ORF) 1a on MERS-CoV. The dsDNA of the target and probes forms a disulfide-induced long self-assembled complex, which protects AuNPs from salt-induced aggregation and transition of optical properties. This colorimetric assay could discriminate down to 1 pmol/μL of 30 bp MERS-CoV and further be adapted for convenient on-site detection of other infectious diseases, especially in resource-limited settings.

Developing a fluorometric aptasensor based on carbon quantum dots and silver nanoparticles for the detection of adenosine

A new simple fluorometric aptasensor was developed for the determination of adenosine (AD) in human urine samples. A mixture of new and green nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) as fluorescents and silver nanoparticles (AgNPs) as quenchers were utilized for aptasensing. The binding of AD to anti-AD aptamer (AP) provided greater protection for unmodified AgNPs against salt-induced aggregation than AP alone. In this case, the fluorescence signal of N,S-CQDs was quenched by the dispersed AgNPs based on the inner filter effect. In the absence of AD, AgNPs tended to aggregate in a high salt solution. Hence, the fluorescence spectrum of N,S-CQDs no longer overlapped with the absorption spectrum of the aggregated AgNPs, and consequently N,S-CQDs did not quench. The linear range of the method was in the range of 50–200 nM, and the limit of detection was 13 nM. The relative standard deviations ranged from 5.2 to 7.3% (n = 6). In addition, the method had high selectivity toward AD in the presence of other interferences. The applicability of the method was examined by the quantitation of AD in urine samples of two patients through the standard addition method with recoveries in the range of 89–93%.

An aptamer based aggregation assay for the neonicotinoid insecticide acetamiprid using fluorescent upconversion nanoparticles and DNA functionalized gold nanoparticles.

An acetamiprid-binding aptamer (ABA), gold nanoparticles (AuNPs) and upconversion nanoparticles (UCNPs) are used in a colorimetric and fluorometric method for the ultrasensitive and selective detection of the pesticide acetamiprid. The ABA is first configured into a duplex with a complementary DNA covalently attached to AuNPs. The resulting dsDNA-functionalized AuNP probe is not stable in 0.15M NaCl solution and aggregates. This causing the color to change from red to purple. In the presence of acetamiprid, the ABA undergoes a structural switch from a DNA duplex to an aptamer-acetamiprid complex and consequently dissociates from the AuNPs. The partially unhybridized AuNPs are stable against salt-induced aggregation and show red color. The ratio of absorbances at 524nm (red) and 650nm (purple blue) varies with the concentration of acetamiprid in the 0.025-10μM concentration range. The colorimetric signal can be further amplified by introducing DNA-modified carboxylated UCNPs (silica-coated NaYF4:Yb,Er) which display red and green fluorescence under 980nm excitation. An inner filter effect occurs between DNA-modified UCNPs and dsDNA-modified AuNPs. The fluorometric assay is based on the measurement of the ratio of red (654nm) and green (540nm) fluorescence and works in the 0.025 to 1μM acetamiprid concentration range and has a 0.36nM detection limit (at a signal-to-noise ratio of 3). Because of the specificity of the aptamer, the assay is high selective. It was successfully used to quantify acetamiprid in contaminated real samples. Graphical abstract Schematic presentation of an upconversion fluorescent assay for acetamiprid. It involves the principle of analyte-triggered structural switch of aptamers, salt-induced AuNP aggregation, and signal amplification from UCNP.

A colorimetric aptasensor for selective detection of oxytetracycline in milk, using gold nanoparticles and oxytetracline-short aptamer

Objective (s): In light of misuse of antibiotics in animal husbandry and their side effects on human health, there is an argent need to develop simple and rapid methods for determining the quantification of antibiotics in biological systems. Materials and Methods: In this work a facile and ultrasensitive colorimetric aptasensor was reported for detection of oxytetracycline (OTC) in water and milk samples employing OTC-short aptamer and gold nanoparticles (AuNPs). Results: In the presence of OTC, the interaction between OTC and its aptamer leads to the separation of OTC aptamer from the surface of AuNPs which is followed by the aggregation of AuNPs by salt, showing an evident color change from red to blue. On the contrary, in the absence of OTC, the attachment of aptamer on the surface of AuNPs can protect AuNPs against salt-induced aggregation with a wine-red color. The proposed aptasensor exhibits excellent sensitivity for detection of OTC with linear range between 20 to 2000 nM with limit of detection (LOD) as low as 10 nM. Furthermore, this strategy was applied to detect OTC in spiked milk samples and presented satisfying linear range from 25 to 1500 nM with the LOD of 20 nM. Conclusion: Owing to demonstrating appropriate sensitivity and selectivity, the designed biosensor can be considered as a promising tool to be applied in the field of biomedicine and food safety.

Regulation of the Stability of Titania Nanosheet Dispersions with Oppositely and Like-Charged Polyelectrolytes.

Charging and aggregation processes of titania nanosheets (TNS) were extensively studied in the presence of oppositely charged or like-charged polyelectrolytes in aqueous dispersions. The surface charge of the TNS was systematically varied by the pH; therefore, positive nanosheets were obtained at pH 4 and negative ones at pH 10. Strong adsorption of poly(styrene sulfonate) (PSS) of high negative line charge density on the TNS was observed at pH 4, leading to charge neutralization and reversal of the original sign of charge of the nanosheets. The adsorption of like-charged poly(diallyldimethylammonium chloride) (PDADMAC) was also feasible through a hydrophobic interaction. The predominating interparticle forces were mainly of the DLVO-type, but additional patch-charge attraction also took place in the case of PSS at low surface coverage. The TNS was found to be hydrophilic at pH 10 and no adsorption of like-charged PSS was possible because of strong electrostatic repulsion between the polyelectrolyte and the surface. The PDADMAC showed high affinity to the oppositely charged TNS surface in alkaline dispersions, giving rise to neutral and positively charged nanosheets at appropriate polyelectrolyte doses. Formation of a saturated PDADMAC layer on the TNS led to high resistance against salt-induced aggregation through the electrosteric stabilization mechanism. These results shed light on the importance of polyelectrolyte concentration, ionic strength, and charge balance on the colloidal stability of TNS, which is especially important in applications, where the nanosheets are dispersed in complex solution containing polymeric compounds and electrolytes.

Elucidating fentanyls differentiation from morphines in chemical and biological samples with surface‐enhanced Raman spectroscopy

Fentanyl and morphine are opioid drugs as well as new psychoactive substances. Even originally introduced as efficient anesthetic drugs to relieve moderate-to-severe pain in clinic, the overdose of new synthetic opioids is currently a serious public health problem in numerous countries worldwide. The entire category of fentanyls has been included in the regulatory list in several countries. There is a great and urgent demand to rapidly recognize fentanyls and morphines in various samples. Here, we report an on-site surface-enhanced Raman spectroscopic method to classify fentanyls from morphines by the Raman spectroscopic signature of the molecular scaffold structure, with an assistance of principle component analysis algorithm. Moreover, by simple but fine-tuning approach of inorganic salt-induced aggregation of gold nanoparticles substrate, we achieved a selective detection of 10ng/mL fentanyl from 2000-fold of heroin, the most common coexisting substance in chemical samples. Good differentiation of 50ng/mL fentanyl from 10000-fold morphine as a main metabolite of heroin in urine samples was also possible after a feasible pretreatment by StageTip procedures. Depending on different structures, the detection sensitivity of five fentanyls ranged from 50 to 2000ng/mL.

Spherically Directed Synthesis and Enhanced Cellular Internalization of Metal-Crosslinked DNA Micelles

Using spherically directed reduction of metal ions, a facile and universal method for in situ crosslinking of DNA micelles is described. By incorporating a series of specific template domains into lipid-DNA monomers, copper-, silver-, and gold-crosslinked DNA micelles are formed. By combining characterization results and simulation algorithms, accurate structural profiling of DNA micelles is achieved for the first time. The prepared metal-crosslinked DNA micelles show obvious merits, including one-step formation of core structure and ligand corona, facile size tunability, monodispersity, stability against salt-induced aggregation, high utilization of DNA ligand, mild synthesis conditions, and less time consumption and better internalization. Based on our spherically directed synthesis, metal-crosslinked DNA micelle flares were further prepared for effective intracellular imaging by integrating an aptamer-toehold strategy. In addition, we extended the strategy to cholesterol-based double-stranded DNA micelles, giving strong evidence that our method is not only universal but also flexible.

Aptamer-based lateral flow assay for point of care cortisol detection in sweat

A new aptamer-based lateral flow strip assay has been designed and developed for on-site rapid detection of cortisol in sweat. Cortisol in sweat has been identified as a key biomarker to monitor physiological stress. A highly sensitive and specific cortisol sensor was achieved by conjugating cortisol-selective aptamers to the surface of gold nanoparticles (AuNPs). Aptamer-functionalized AuNPs are stable against salt-induced aggregation. When cortisol molecules are present in the sample, they interact with the designed aptamers causing their desorption from the AuNP surface. Free AuNPs can then be captured by reaction with cysteamine immobilized on the test zone of the lateral flow test strip. This enables the visual detection of cortisol within minutes. Important parameters that affect the detection sensitivity in both solution and lateral flow assays, such as the loading density of aptamers per AuNP, salt and cysteamine concentrations, were investigated to provide the optimum assay performance. This hand-held device successfully exhibited a visual limit of detection of 1 ng/mL, readily covering the normal range of free cortisol in sweat (8–140 ng/mL). No significant cross reactivity to other stress biomarkers was observed. The advantages of this paper-based biosensor over previously reported test strips include the use of aptamers (which are more stable, simpler to use and lower cost than antibodies) and a simplified lateral flow assay (LFA) strip design (without the use of complementary aptamers in the test line). The resulting LFA aptasensor provides a rapid, sensitive, user-friendly and cost-effective point of care device for cortisol detection in sweat and other biofluids.

Colorimetric detection of PCA3 in urine for prostate cancer diagnosis using thiol-labeled PCR primer and unmodified gold nanoparticles

BackgroundPCA3, a non-coding RNA, has been approved as a potential urinary biomarker for prostate cancer. However, PCA3 urine tests have some limitations. Therefore, we developed a colorimetric method for PCA3 detection in urine. MethodsThe assay was based on interactions between unmodified gold nanoparticles (AuNPs) and thiolated PCR products. Thiolated PCR products were amplified by RT-PCR using a thiol-labeled primer at the 5′ end. Thiolated products of PCA3 bound to the surface of AuNPs and led to the prevention of salt-induced aggregation (red color). In the absence of the PCR products, AuNPs changed their color from red to blue due to the salt-induced aggregation. These changes were detected by the naked eye and spectrophotometer. ResultsOur assay was specific for PCA3 in prostate cancer cell lines with a visual detection limit of 31.25 ng/reaction. The absorption ratio 520/640 nm was linear against PCR product concentration (R2 = 0.9798) in the reaction. This method is promising for discrimination of prostate cancer patients from both healthy controls and benign prostatic hyperplasia patients according to their urinary PCA3 expression levels. ConclusionsThis study established a simple, rapid, sensitive and specific assay for PCA3 detection which may be applicable for prostate cancer diagnosis.

Colorimetric adenosine aptasensor based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles

An aptamer based assay is described for the colorimetric detection of adenosine. The presence of adenosine triggers the deformation of hairpin DNA oligonucleotide (HP1) containing adenosine aptamer and then hybridizes another unlabeled hairpin DNA oligonucleotide (HP2). This leads to the formation of a double strand with a blunt 3' terminal. After exonuclease III (Exo III)-assisted degradation, the guanine-rich strand (GRS) is released from HP2. Hence, the adenosine-HP1 complex is released to the solution where it can hybridize another HP2 and initiate many cycles of the digestion reaction with the assistance of Exo III. This leads to the generation of a large number of GRS strands after multiple cycles. The GRS stabilize the red AuNPs against aggregation in the presence of potassium ions. If, however, GRS forms a G-quadruplex, it loses its ability to protect gold nanoparticles (AuNPs) from salt-induced AuNP aggregation. Therefore, the color of the solution changes from red to blue which can be visually observed. This colorimetric assay has a 0.13nM detection limit and a wide linear range that extends from 5nM to 1μM. Graphical abstract Schematic presentation of acolorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles.

2-Step lectin-magnetic separation (LMS) strategy combined with AuNPs-based colorimetric system for S. aureus detection in blood

A novel 2-step lectin-magnetic separation (LMS) method combined with gold nanoparticle (AuNPs) -based colorimetric system for specifically and quantitatively detecting S. aureus in blood samples was established. In this study, wheat germ agglutinin lectin from Triticum vulgaris (wheat) (WGA) was applied for the first time as a recognition agent in a 2-step magnetic separation method using magnetic nanoparticles (MNPs) for effective and low-cost bacteria enrichment. A quantitative detection strategy based on the long dsDNA amplicons stabilized with AuNPs against salt-induced aggregation was investigated. The 2-step LMS method resulted in a high enrichment efficiency even at low concentrations (100–102 CFU/mL) of S. aureus in pure culture and blood samples. The AuNPs-based colorimetric system was applied for quantitative detection of S. aureus with a limit of detection (LOD) at 3.5 × 100 CFU/mL. The complete process starting with the blood samples treatment, 2-step LMS, and AuNPs-based colorimetric detection took only 7 h. The low cost and sensitive 2-step LMS combined with AuNPs-based colorimetric detection holds considerable potential for early sepsis diagnosis.

Label-free aptasensors based on fluorescent screening assays for the detection of Salmonella typhimurium

We report two label-free fluorescent aptasensor methods for the detection of S. typhimurium. In the first method, we have used a ‘‘turn off’’ approach in which the aptamer is first intercalated with SYBR Green I (SG), leading to a greatly enhanced fluorescence signal. The addition of S. typhimurium (approximately 1530–96938 CFU/mL), which specifically binds with its aptamer and releases SG, leads to a linear decrease in fluorescence intensity. The lowest detection limit achieved with this approach was in the range of 733 CFU/mL. In the second method, a ‘‘turn on’’ approach was designed for S. typhimurium through the Förster resonance energy transfer (FRET) between Rhodamine B (RB) and gold nanoparticles (AuNPs). When the aptamer and AuNPs were mixed with RB, the fluorescence of RB was significantly quenched via FRET. The aptamer adsorbs to the AuNP surface to protect them from salt-induced aggregation, which leads to the fluorescence quenching of RB in presence of AuNPs. Upon the addition of S. typhimurium, S. typhimurium specifically binds with its aptamer and loses the capability to stabilize AuNPs. Thus, the salt easily induces the aggregation of AuNPs, resulting in the fluorescence recovery of the quenched RB. S. typhimurium concentrations ranging from 1530 to 96938 CFU/mL with the detection limit of 464 CFU/mL was achieved with this methodology. Given these data, some insights into the molecular interactions between the aptamer and the bacterial target are provided. These aptasensor methods also may be adapted for the detection of a wide variety of targets.

Detection of chloramphenicol using a novel apta-sensing platform based on aptamer terminal-lock in milk samples

In this paper, a novel apta-sensing colorimetric platform for rapid detection of chloramphenicol (CAP) in raw milk was developed. The AuNPs are stabilized by short-sequences aptamers against salt induced aggregation and this is the base of most colorimetric aptasensors development. However, the statute shows low sensitivity for the long-sequence aptamers. Herein, we propose an alternative strategy that use intact long-sequence aptamers for develop a highly sensitive AuNP-based colorimetric aptasensor. Determination of CAP in animal derived foods is an urgent demanded in the effort to minimize food safety risk. Therefore, we chose it as the representative model to construct the colorimetric sensing platform based on aptamer terminal-lock (ATL). In the ATL, intact aptamer was used as a molecular recognition element and a short-sequence oligonucleotide serving as a locker probe (LP) which is complementary of aptamer terminal fragments. By formation of aptamer/target complex, the LP leaves the ATL and adsorbs on the surface of AuNPs, leading to the AuNPs stabilization against salt-induced aggregation. This aptasensor shows a low limit of detection (0.03 nM) with high selectivity toward CAP. Moreover, the designed sensing platform was successfully applied to detect CAP in the milk samples. These results demonstrate our introduced label-free method for CAP detection is simple, sensitive, and highly selective.

A terbium-based metal-organic framework@gold nanoparticle system as a fluorometric probe for aptamer based determination of adenosine triphosphate.

This study reports on a method for fluorometric aptasensing of adenosine triphosphate (ATP). It is based on the interaction of dispersed (red) and agglomerated (blue) gold nanoparticles (AuNPs) with a water-dispered terbium(III) based metal-organic framework (Tb-MOF). The dispersed AuNPs quench the emissions of the Tb-MOF, while the aggregated AuNPs have little effect. Under the condition of high salt concentration, the free aptamer against ATP does not stabilize the AuNPs against aggregation. This causes a color change from red to blue and weak quenching of the fluorescence of the Tb-MOF (with peaks at 489nm and 544nm after excitation at 290nm). On addition of ATP, it will be bound by its aptamer to form a complex that is adsorbed on the AuNPs. This protects the AuNPs from salt-induced aggregation and the color (with a peak at 525nm) remains red. The two fluorescence bands of the Tb-MOF are therefore suppressed by fluorescence resonance energy transfer (FRET) between Tb-MOF and the dispersed AuNPs. Fluorescence drops linearly in the 50nM to 10μM ATP concentration range, and the detection limit is 23nM. ATP analogs such as guanosine triphosphate, uridine triphosphate, cytidine triphosphate, adenosine monophosphate and cyclic adenosine monophosphate have no obvious interference. The method was successfully applied to the determination of ATP in (spiked) human plasma samples and gave satisfactory recoveries. Graphical abstract Schematic of a terbium-based metal-organic framework@gold nanoparticle system as a fluorometric probe for aptamer based determination of adenosine triphosphate. The dispersed gold nanoparticles (AuNPs) quench the fluorescence of the terbium-based metal-organic framework (Tb-MOF), while the aggregated AuNPs have little effect. In the presence of adenosine triphosphate (ATP), the aptamer-ATP complexes provide greater protection towards AuNPs than aptamer alone under high salt condition. Based on this, a novel Tb-MOF@AuNP platform is established for ATP detection.

Colorimetric detection of DNA at the nanomolar level based on enzyme-induced gold nanoparticle de-aggregation.

The authors describe a colorimetric method for the determination of DNA based on the deaggregation of gold nanoparticles (AuNPs) induced by exonuclease III (Exo III). DNA amplification is accomplished by Exo III to generate large quantities of the residual DNA. Residual DNA tethers onto the surfaces of AuNPs which prevents their aggregation. Hence, the color of the solution is red. However, in the absence of DNA, salt-induced aggregation is not prevented, and the bluish-purple color of the aggregated AuNPs is observed. The ratio of absorbances at 525 and 625nm increases up to 150nM DNA concentrations, and the LOD is as low as 3.0nM. It is shown that the presence of 300nM concentrations of random DNA (with a mass up to 10-fold that of target DNA) does not interfere. The method was successfully applied to the analysis of DNA in spiked serum samples. The method is simple, reliable, and does not require complicated amplification steps and expensive instrumentation. Graphical abstract Schematic of a sensing strategy for DNA detection by exonuclease III-induceddeaggregation of gold nanoparticles. DNA concentrations as low as 3 nM can be detectedvia colorimetric monitoring of the color change from red to purple-blue.

Functionalized Titania Nanosheet Dispersions of Peroxidase Activity

Nanocomposites of titania nanosheets (TNS), horseradish peroxidase (HRP), and poly(diallyldimethylammonium chloride) (PDADMAC) were prepared, and their colloidal and functional stabilities were assessed. HRP quantitatively adsorbed on bare TNS, and the adsorption process did not affect the charging and aggregation behavior of the colloidal system. The obtained TNS–HRP composite was functionalized by PDADMAC to stabilize the enzyme on the surface and to maintain good colloidal stability. Depending on the PDADMAC dose applied, its adsorption on TNS–HRP led to charge reversal of the particles from negative to positive. The formation of a saturated polyelectrolyte layer on the TNS–HRP (TNS–HRP–PDADMAC) gave rise to highly stable colloids, and especially the resistance against salt-induced aggregation was excellent. The enzymatic activity of different systems was investigated as a function of the pH of the medium and over time. The results indicated that HRP remained enzymatically active upon immobilization, a...

Colorimetric and energy transfer based fluorometric turn-on method for determination of microRNA using silver nanoclusters and gold nanoparticles.

The authors describe a dual (colorimetric and fluorometric) detection scheme for microRNA. The method is based on the use of fluorescent DNA-modified silver nanoclusters (DNA-AgNCs) and gold nanoparticles (AuNPs). The DNA-AgNCs were linked to AuNPs via the interaction of ssDNA-AgNC probes with AuNPs. This led to quenching of the fluorescence of the DNA-AgNCs (best measured at excitation/emission peaks of 370/450nm) and also prevents salt-induced aggregation of the AuNPs. Upon addition of microRNA, the DNA on the AgNCs hybridizes with microRNA. This led to the formation of a DNA-AgNC/miRNA hetero-duplex and increase of the distance to the AuNPs. Quenching was suppressed and fluorescence was restored. The presence of microRNA also affected salt-induced particle aggregation as reflected by a color change from red to purple that is visible with bare eyes. The visual detection limit for microRNA is 0.6nM, and the fluorometric detection limit is 0.4 pM. Graphical abstract Schematic of thecolorimetric and fluorometric assay for microRNA (miRNA). It is based on the use offluorescent DNA-modifiedsilver nanoclusters (AgNCs) and gold nanoparticles (AuNPs).

(Invited) Variance Spectroscopy Studies of Single-Walled Carbon Nanotube Aggregation

Understanding aggregation in samples of dispersed, well-individualized single-walled carbon nanotubes (SWCNTs) is important for many basic studies and applications. We will describe a study that uses the recently developed technique of variance spectroscopy to monitor aggregation kinetics of SWCNTs in aqueous surfactant dispersions. Unlike other techniques used to investigate colloidal dispersions, variance spectroscopy resolves individual subpopulations in mixed samples based on their distinct fluorescence signatures. Following the addition of NaCl coagulant to unsorted SWCNT dispersions, we monitored changes in the independent particle concentrations of six semiconducting (n,m) species. These data were supplemented by time-resolved measurements of fluorescence intensity, which showed pseudo-first order kinetics that was independent of SWCNT structure and was attributed to quenching by energy transfer to metallic SWCNTs. The decreases in particle concentration directly revealed aggregation. They were found to fit reasonably well to a second-order rate law with some dependence on structure at higher salt concentrations. Finally, covariance analysis was applied to the variance data to monitor correlated spectral fluctuations arising from specific heteroaggregates. Those results showed equivalent associations between different semiconducting SWCNT species during salt-induced aggregation.

The mechanism of the adsorption of dsDNA on citrate-stabilized gold nanoparticles and a colorimetric and visual method for detecting the V600E point mutation of the BRAF gene.

A study is presented on the binding kinetics and mechanism of the adsorption of dsDNA on citrate-capped gold nanoparticles (AuNPs). Methods include fluorescence titration, isothermal calorimetry (ITC) titration, dynamic light scattering and gel electrophoresis. It is found that the fluorescence of probe DNA (labeled with Rhodamine Green and measured at excitation/emission peaks of 498/531nm) is quenched by addition of AuNPs. The Stern-Volmer quenching constant (Ksv) is 1.67 × 10^9 L·mol-1 at 308K and drops with increasing temperature. The quenching mechanism is mainly static. The results of both fluorescence titrations and ITC show negative values for ΔH and ΔS values. This shows ion-induced dipole-dipole interaction to be the main attractive forces between dsDNA and AuNPs, while electrostatic interactions result in repulsion. The repulsive forces lead to a lower affinity between dsDNA and AuNPs (compared to single-strand DNA). It is also found that dsDNA can prevent the aggregation of AuNPs which is accompanied by a color change from red into blue. The visual detection limit with bare eyes for dsDNA1 is 36 pM. Based on these findings, a colorimetric method was developed to detect the proto-oncogene of serine/threonine-protein kinase B-Raf V600E point mutation in HT29, Ec109, A549, Huh-7 and SW480 cell lines. Graphical abstract Schematic of the salt-induced aggregation of uncapped gold nanoparticles (AuNPs) which leads to a color change from red to blue. If the AuNPs are coated with dsDNA, aggregation is suppressed.