Photoluminescence Mechanism of DNA ... - ACS Publications

Subscriber access provided by UNIVERSITY OF LEEDS

Article

Photoluminescence Mechanism of DNA-Templated Silver Nanoclusters: Coupling between Surface Plasmon and Emitter and Sensing of Lysozyme Xiaorong Liu, ruoxin Hu, Zhidan Gao, and Na Shao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00589 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 17, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Photoluminescence Mechanism of DNA-Templated Silver Nanoclusters: Coupling between Surface Plasmon and Emitter and Sensing of Lysozyme Xiaorong Liu, Ruoxin Hu, Zhidan Gao, and Na Shao* College of Chemistry, Beijing Normal University, Beijing 100875, PR China *Corresponding author: E-mail: [email protected] Fax: +86-10-58802146

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

ABSTRACT:

DNA-templated silver nanoclusters (DNA-AgNCs) have now been thrust into the limelight with their superior optical properties and potential biological applications. However, the origin of photoluminescence from DNA-AgNCs still remains unclear. In this work, DNA-AgNCs were synthesized and the photoluminescence properties as well as the biosensing applications of the designed DNA-AgNCs were investigated. The photoluminescence properties of the DNAAgNCs were studied under three regions of excitation wavelength based on the UV-Visible absorption spectra. It was deemed that the photoluminescence originated from coupling between the surface plasmon and the emitter in AgNCs when they were excited by visible light above 500 nm, and thus the emission wavelength varied with changing the excitation wavelength. The photoluminescence of the red emitting only AgNCs was the intrinsic fluorescence when excited from 200 to 400 nm, which was only related to the emitter; But for two component of purple and red emitting AgNCs, the emission wavelength varied with the excitation wavelength ranging from 300-360 nm, and the photoluminescence was a coupling between the surface plasmon and the emitter. The photoluminescence was only related to the surface plasmon when the AgNCs were excited from 400 nm to 500 nm. Four DNA probes were designed and each contained two parts: one part was the template used to synthesize AgNCs and it was same to all, and the other part was the lysozyme binding DNA (LBD) used to bind lysozyme and two kinds of LBD were studied. It was deemed that the difference in DNA bases, sequence and secondary structure caused the synthesized DNA-AgNCs to be different in photoluminescence properties and sensing ability to lysozyme. And the sensing mechanism based on photoluminescence enhancement was also presented. This work explored the origin of photoluminescence and the sensing ability of


2

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

DNA-AgNCs, and is hoped to make a better understanding of this kind of photoluminescence probe.

INTRODUCTION By combining the virtues of materials chemistry and nanotechnology, researchers have recently developed few-atom, molecular-scale noble metal nanoclusters (NCs) as optical-active nanomaterials for the construction of optical biosensors.1,2 Metal NCs own an appealing set of features, such as excellent photostability, subnanometer size, and low toxicity, which could complement the properties of organic dyes and semiconductor quantum dots (QDs).3,4,5 Among them, AgNCs have more potentials in biological applications6-9 due to simple synthesis and good biocompatibility. To date, various kinds of scaffold have been employed to synthesize AgNCs, such as DNA,1017

thiols,18-20 polymers,21,22 peptides and proteins.23-25 As the scaffold for preparing metal NCs,

DNA oligonucleotides have been widely used. In particular, the high affinity of siver ions to cytosine bases on single-stranded DNA favors DNA oligonucleotides as an excellent template for the synthesis of fluorescent AgNCs.26-27 Since Dickson’s group reported the synthesis of AgNCs using a 12-mer DNA strand as the template,28 a series of DNA-AgNCs with fluorescence spectra ranging from UV to near IR scopes have been synthesized successfully by changing the sequence,10-12 length of strands,13-15 and secondary structure16,17 of the DNA scaffold. The excellent properties, such as tunable emission, high quantum efficiency and good biocompatibility of the DNA-AgNCs motivated more research on them. However, several issues fundamental to the photoluminescence of AgNCs are still not well understood. For example, an interesting phenomenon, that is the emission wavelength varied


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

with the excitation wavelength under visible light excitation, has been reported by many researchers. 29-32 Diez et al29 deemed that there were three components in the sample which led to the formation of three groups of peaks. Richie30 and many other researchers31-33 proposed the size of AgNCs fit a poission distribution and the components of AgNCs were inhomogeneous. Recently, Zhang and coworkers attributed this phenomenon to the coupling between the surface plasmon and emitter in AgNCs, 34 which aroused our great interest. In this work, the photoluminescence origin and the sensing ability of four designed DNAAgNCs as well as the sensing mechanism were explored. It was found that under visible light excitation above 500 nm, the dependence of emission wavelength on excitation wavelength could be attributed to the strong coupling between the surface plasmon and the emitter in DNAAgNCs. When the DNA-AgNCs were excited by ultraviolet light, the intrinsic fluorescence generated and the emission wavelength was independent on the excitation wavelength for red only emitting AgNCs, but the emission wavelength was dependent on the excitation wavelength for two components of purple and red emitting AgNCs. When the DNA-AgNCs were excited from 400 nm to 500 nm, the photoluminescence was only related to the surface plasmon and the emission wavelength depended on the excitation wavelength. Furthermore, four DNA probes which have potential sensing ability for lysozyme were designed. It was found the sequence as well as the secondary structure of DNA probes could affect the photoluminescence properties of the synthesized AgNCs and the sensing ability of the designed DNA-AgNCs to lysozyme. And the sensing mechanism of DNA-AgNCs to lysozyme based on photoluminescence enhancement was presented. This study was hoped to be helpful for the understanding of the photoluminescence origin of DNA-AgNCs. EXPERIMENTAL


4

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

2.1 Reagents and materials Sodium borohydride (NaBH4, 96%) was purchased from Sinopharm Chemical Company. Silver nitrate (AgNO3) was purchased from Tianjin Heowns Biochem Technologies LLC. Lysozyme, Bovine serum anbumin (BSA), thrombin, trypsin and immunoglobin G (IgG) were purchased from Beijing Dingguo Biotechnology Company. All other reagents were of analytical reagent grade and used as received without further purification. All the stock solutions and buffer solutions were prepared using ultrapure water which was obtained through a Millipore Milli-Q water purification system and had an electric resistance over 18.0 MΩ. All DNA strands with specific sequence were synthesized by Beijing Dingguo Biotechnology Company. The concentration of DNA was calculated from the absorbance of 260 nm. The DNA probes used in our study were as following: DNA1, 5′-ATC TAC GAA TTC ATC AGG GCT AAA GAG TGC AGA GTT ACT TAG AAA AAC CCT TAA TCC CC-3′; DNA2, 5′-CCC CTA ATT CCC AAA AAA TCT ACG AAT CAT CAG GGC TAA AGA GTG CAG AGT TAC TTA G-3′; DNA3, 5′-GCA GCT AAG CAG GCG GCT CAC AAA ACC ATT CGC ATG CGG CAA AAA CCC TTA ATC CCC-3′; DNA4, 5′-CCC CTA ATT CCC AAA AAG CAG CTA AGC AGG CGG CTC ACA AAA CCA TTC GCA TGC GGC-3′; DNA5, 5′-ATC TAC GAA TTC ATC AGG GCT AAA GAG TGC AGA GTT ACT TAG AAA AAC CCT CTT AAC CC3′; DNA6, 5′-GCA GCT AAG CAG GCG GCT CAC AAA ACC ATT CGC ATG CGG C CCC TTA ATC CCC-3′; DNA7, 5′-CCC CTA ATT CCC G CAG CTA AGC AGG CGG CTC ACA AAA CCA TTC GCA TGC GGC-3′. And two control DNA probes were used: Control 1, 5′ATC TAC GAA TTC GTA AAT CGT CGA CAG GAA TTG GCG GGC CGG AAA AAC CCT TAA TCC CC-3′; And control 2, 5′-ATC TAC GAA TTC GAA TTG CGA CAG TCG


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

GGA CAT GTC GCG AGG AAA AAC CCT T AA TCC CC-3′. Among these, the underlined sequence 5′-CCCTTAATCCCC-3′ and 5′-CCCTCTTAACCC-3′ were the scaffold for synthesis of AgNCs. The sequence AAAAA was the linker. 5′-ATC TAC GAA TTC ATC AGG GCT AAA GAG TGC AGA GTT ACT TAG-3′ was the DNA template of lysozyme RNA aptamer selected by Ellington, 35 which was widely used in many research36-39 and proved to have high binding affinity to lysozyme (here, it was called LBD1). 5′-GCA GCT AAG CAG GCG GCT CAC AAA ACC ATT CGC ATG CGG C-3′ was the lysozyme binding DNA aptamer (LBD2) selected by Lammertyn40 and used by some researchers.41-44 2.2 Instrumentation UV-Visible absorption spectra were carried out on a TU-1901 diode-array spectrophotometer. Photoluminescence (PL) spectra were obtained using a Hitachi F-4600 Fluorescence Spectrophotometer. Time-resolved fluorescence measurements were carried out on an OB920 single-photon counting fluorometer. Quantum yields were detected using FluoroMax-4 Spectrofluorometer. Circular dichroism (CD) spectra were detected by Applied Photophysics Chirascan 250. Transmission electron microscopy (TEM) graphs were performed using a TF20 (FEI). The pH was determined by a REX PHS-3C pH meter. 2.3 Preparation of DNA-AgNCs DNA-AgNCs were synthesized according to the previous report13 with slight modification. The DNA probes (100 µM) were first heated at 75 ℃ in sodium phosphate buffer (PB, 20 mM, pH=6.6) for 10 min, and then cooled down to room temperature. AgNO3 and the designed DNA probes were used as the precursor and stabilizer respectively. Briefly, AgNO3 (20 µL, 3.6 mM)


6

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

was mixed with DNA solution (120 µL, 100 µM) in PB buffer (pH=6.6, 1840 µL, 20 mM), and the mixtures were incubated at room temperature for 20 min. Then fresh NaBH4 (20 µL, 3.6 mM) was added and the resulted solution was kept in dark for 40 min to form DNA-AgNCs. The molar ratio of Ag+: DNA: NaBH4 was 6:1:6. The concentration of DNA-AgNCs was referred to the concentration of DNA unless mentioned otherwise. 2.4 Photoluminescence measurement of DNA-AgNCs probe itself and sensing of lysozyme The measurements of photoluminescence spectra, time-resolved fluorescence and the absolute quantum yields of the four designed DNA-AgNCs were all conducted using the as-synthesized DNA-AgNCs probes with a concentration of 6 µM in 20 mM sodium phosphate buffer (PB). For sensing of lysozyme, the as-synthesized DNA-AgNCs solutions were diluted 10 times with a final concentration of 600 nM. A stock solution of lysozyme (100 µM) was prepared in deionized water and diluted when needed. To be brief, The as-prepared DNA-AgNCs (80 µL, 6 µM) were mixed with equal volume (80 µL) of lysozyme solution with different concentration, sodium phosphate buffer (80 µL, 20 mM) and water (560 µL) to bring the final volume to 800 µL. The mixed solution was incubated for 15 min under 37 ℃ water bath, and then the photoluminescence spectra of DNA-AgNCs were recorded. The sensing of lysozyme of the four designed DNA-AgNCs was under the same conditions mentioned above. 2.6 Circular dichroism (CD) measurement The circular dichroism (CD) spectra of DNA-AgNCs for sensing of lysozyme were detected as below. The as-prepared DNA-AgNCs (240 µL, 6 µM) in PB (240 µL, 20 mM) were mixed with deionized water (240 µL) or different concentration of lysozyme, and then 1680 µL water


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

was added to make the final volume to 2400 µL. After 15 min’s incubation under 37 ℃ water bath, the circular dichroism (CD) spectra were recorded. RESULTS AND DISCUSSION 3.1 Characterizing the photoluminescence origin of DNA-AgNCs The formation of DNA-AgNCs was first verified and characterized by UV-Visible absorption spectroscopy and photoluminescence spectroscopy. The evolution of the absorption spectra and photoluminescence spectra of DNA-AgNCs was monitored during synthesis. DNA1-AgNCs and DNA4-AgNCs were taken as examples. As shown in Fig 1(A) (DNA4-AgNCs in Fig S1(A)), there was no absorption peak between 300 nm and 650 nm before the addition of NaBH4 to the solution of DNA1+AgNO3 (curve a and b). After adding NaBH4, the absorption spectra presented an obvious evolution immediately and a broad absorption band from 300 nm to 500 nm appeared (curve c). As the reaction time evolved to 2 and 4 minutes, two obvious peaks at 350 nm and 435 nm appeared (curve d and e). After 6 minutes and later, the two peaks of 350 nm and 435 nm became stronger and a weak absorption band centered at 555 nm appeared (curve f, g and h). The corresponding photoluminescence spectra of DNA1-AgNCs when excited at 555 nm were shown in Fig 1(B) (DNA4-AgNCs in Fig S1(B)). It can be seen that the photoluminescence intensity increased sharply at the very beginning (curve c) and then increased gradually after 2 minutes during the synthesis (curve d, e, f, g and h).


8

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 1. (A) UV-Visible absorption spectra and (B) corresponding photoluminescence spectra (λex = 555 nm) of DNA1, DNA1+Ag+, and freshly prepared DNA1-AgNCs at various reaction time. (a, DNA; b, DNA+Ag+; c-h, DNA+Ag++NaBH4 after 20 s, 2 min, 4min, 6 min, 10 min, and 20 min, respectively.) (C) Photoluminescence spectra of DNA1-AgNCs under different excitation wavelength. On the origin of fluorescence of NCs, quantum confinement effect of metal core with size below 2 nm was always considered. However, many nanoparticles with size larger than 2 nm can also fluoresce, 45,46 which indicated that fluorescence can’t only be attributed to the small size, and ligand’s role should also be emphasized which had been reported by several researchers. 47.48 In DNA-AgNCs, the ligand-to-metal charge transfer (LMCT) from the electron-rich N atom in DNA to AgNCs metal core should be regarded as the origin of photoluminescence.29,34,49,50 When DNA1-AgNCs were excited at 350 nm or 555 nm, photoluminescence at 625 nm could be observed, as shown Fig 1(C) (DNA4-AgNCs in Fig S1(C)), and it was speculated that the absorption peaks centered at 350 nm and 555 nm were LMCT absorption bands. While, no photoluminescence at 625 nm could be observed when DNA1-AgNCs were excited at 435 nm, and it was considered the absorption band centered at 435 nm should be the surface plasmon absorption, which exists in nanoparticles larger than 2 nm. 51-54


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

Fig 2. (A) Emission spectra as a function of excitation wavelength (From left to right, the excitation wavelength was from 440 nm to 650 nm with 10 or 20 nm interval) and (B) Emission energy (EL) as a function of excitation energy of DNA1-AgNCs. Solid red dots and solid blue squares represented the experimental data. Solid red curve showed the theoretical data calculated according to the Kretschmann geometry and solid blue line was the fitting line. According to Kasha’s role, fluorescence occurs in appreciable yield only from the lowest excited state of a given multiplicity. Therefore, the emission wavelength should be irrelevant to excitation wavelength. As shown in Fig S2 (Fig S2(A) for DNA1-AgNCs, and Fig S2(B) for DNA4-AgNCs), by varying excitation wavelength over the near-UV range (250-400 nm), the corresponding fluorescence emission wavelength didn’t show any dependence on the excitation wavelength, and the photoluminescence intensity was the highest when excited at 350 nm. It was deemed that the photoluminescence when excited near 350 nm was intrinsic fluorescence of DNA-AgNCs originated from the emitter only. However, it can be seen from Fig 2(A) (DNA4AgNCs in Fig S3(A)) that the emission wavelength varied with changing the excitation wavelength from 440 nm to 650 nm. And the photoluminescence spectra could be divided into two groups; one was the spectra when excited from 440 nm to 490 nm, while the other was that


10

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

excited from 500 nm to 650 nm. When the DNA1-AgNCs were excited at 555 nm, the photoluminescence intensity at 625 nm was the strongest. As is known to all, noble metal nanoparticles, such as Au and Ag, can produce surface plasmon.55-57 And many reports have mentioned the coupling interactions between the surface plasmon and the emitter in semiconductors.58-60 The coupling is mediated via a dipole interaction, which exists between the electric field produced by the surface plasmon and the electric dipole moment of the emitter transition. According to Kretschmann geometry, the expression for the states resulting from the surface plasmon-emitter interaction gives:

EL

[E (λ ) =

pl

(λ ) + E 0 2

]±

2

∆

[E +

pl

(λ ) − E 0

]

2

4

Equation (1)

Here, Epl(λ) was the energy of uncoupled surface plasmon and was calculated by Epl(λ) = 1240/λ, λ was the excitation wavelength, with Epl in eV and λ in nm. E0 was the LMCT transition energy (2.234 eV, 555 nm for this study) and ∆ was the energy related to the interactions between the photons and electrons, which for the present case was found to be 234 meV. As Fig 2(B) (DNA4-AgNCs in Fig S3(B)) illustrated, there was an excellent linear relationship between emission energy and excitation energy in the spectra region of 440-490 nm (the solid blue squares), which testified the photoluminescence between 440 nm and 490 nm was related to the surface plasmon. However, for the solid red dots, the emission energy was nonlinear with the excitation energy ranging from 500 nm to 650 nm, and the experimental data were in very good agreement with the calculated energies EL (theoretical data, solid red curve) according to equation (1). Therefore, the photoluminescence of the DNA-AgNCs obtained by excitation under


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

500 nm to 650 nm was supposed to originate from the coupling between the surface plasmon and the emitter. The interesting phenomenon, strong dependence of emission wavelength on excitation wavelength, has been reported by many researchers.29-32 Previous reports simply proposed there were multiple components29 or the size of the synthesized AgNCs was inhomogeneous30-33. Recently, Zhang34 proposed this phenomenon to be coupling between surface plasmon and emitter. To verify the coupling mechanism between the surface plasmon and emitter was applicative to other kind of DNA-AgNCs, another DNA template was used and the photoluminescence properties of the thus synthesized DNA5-AgNCs were studied. As shown in Fig S3(C), when DNA5-AgNCs were excited from 300 nm to 600 nm, the emission wavelength varied with changing the excitation wavelength. It can be seen from Fig S3(D) the photoluminescence spectra could be divided into three groups. There was an excellent linear relationship between emission energy and excitation energy in the spectra region from 380 to 500 nm, which testified the photoluminescence excited in this range was only related to the surface plasmon. However, the emission energy was nonlinear with the excitation energy ranging from 300-360 nm and 500-600 nm, and the experimental data were both in accordance with the calculated energies EL (theoretical data) according to Equation (1). Therefore, the photoluminescence of DNA5-AgNCs obtained by excitation under 300-360 nm or 500-600 nm was supposed to originate from the coupling between the surface plasmon and the emitter. The two groups of photoluminescence spectra excited by 300-360 nm and 500-600 nm should come from two types of emitters (Purple and Red), respectively. And in our studies, for DNA1 and DNA4 templated AgNCs, the average lifetime and transmission electron microscopy (TEM) images both demonstrated there was only one


12

Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

component with a definite size in the synthesized DNA-AgNCs. As shown in Fig S4 and Table 1, the average fluorescence lifetime detected under various reduction time was almost the same (DNA1-AgNCs: ~ 2 ns; DNA4-AgNCs: ~ 1.6 ns). This indicated the red only emitting DNA1AgNCs and DNA4-AgNCs were homogeneous. The TEM images in Fig S5 showed that the four synthesized red only emitting DNA-AgNCs were monodispersed. Table 1. Average lifetime of DNA1-AgNCs and DNA4-AgNCs calculated using two exponential decay fitting analysis under various reduction time after adding NaBH4. Reduction Time(min) τ(ns) DNA1-AgNCs

χ2 τ(ns)

DNA4-AgNCs

χ2

2

4

6

8

10

20

1.9

1.9

2.0

2.1

2.1

2.2

1.02 0.99 0.98 1.08 1.00 1.02 1.6

1.5

1.6

1.7

1.7

1.8

0.99 1.09 0.99 1.02 1.10 1.01

3.2 Photoluminescence properties of the four designed DNA-AgNCs and their sensing ability to lysozyme To explore the biosensing application of DNA-AgNCs, four probes which have potential binding ability to lysozyme were designed. For the four DNA probes, each contained two parts. One part was for AgNCs synthesis (AgNCs template) and it was same to all, and the other part was used to recognize lysozyme and two kinds of lysozyme binding DNA (LBD) were used. The photoluminescence spectra of the four DNA-AgNCs were studied and shown in Fig 3(A). It can be seen that the photoluminescence intensity of the four designed DNA-AgNCs was in the order: DNA3-AgNCs > DNA1-AgNCs > DNA2-AgNCs > DNA4-AgNCs, and the maximum emission


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

wavelength of DNA2-AgNCs was about 15 nm red shifted compared with that of the other three. The absolute quantum yield (QY) of the four DNA-AgNCs was also detected, as shown in Fig 3(B). The order of the QY of the four DNA-AgNCs was consistent with the relative photoluminescence intensity in Fig 3(A).

Fig 3. (A) Photoluminescence spectra of the four designed DNA-AgNCs probes. The excitation wavelength was 555 nm. (B) The absolute quantum yield of the four designed DNA-AgNCs. Sequence of DNA probes used in this study: DNA1, 5′-ATC TAC GAA TTC ATC AGG GCT AAA GAG TGC AGA GTT ACT TAG AAA AAC CCT TAA TCC CC-3′; DNA2, 5′-CCC CTA ATT CCC AAA AAA TCT ACG AAT CAT CAG GGC TAA AGA GTG CAG AGT TAC TTA G-3′; DNA3, 5′-GCA GCT AAG CAG GCG GCT CAC AAA ACC ATT CGC ATG CGG CAA AAA CCC TTA ATC CCC-3′; DNA4, 5′-CCC CTA ATT CCC AAA AAG CAG CTA AGC AGG CGG CTC ACA AAA CCA TTC GCA TGC GGC-3′. It was repoted that the length, sequence and secondary structure of DNA were significant for the optical properties of DNA-AgNCs.10-17 For DNA1-AgNCs and DNA2-AgNCs, the QY of DNA1-AgNCs was a little bit higher than that of DNA2-NCs, and the photoluminescence


14

Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

intensity of DNA1-AgNCs was a bit higher than that of DNA2-AgNCs. As shown in Fig S7(A) and (B), the AgNCs template was “free” in DNA1, while it formed base pairs partially in DNA2. During the synthesis of AgNCs, the template part in DNA2 needed to open the paired bases and thus DNA1 should have higher efficiency than DNA2 did for synthesizing AgNCs. Therefore, the photoluminescence intensity was in the order of DNA1-AgNCs > DNA2-AgNCs as shown in Fig 3(B). The circular dichroism (CD) spectra of DNA1 and DNA2 before and after synthesis of AgNCs were shown in Fig S8, indicating that DNA2 would undergo conformational change during the synthesis of AgNCs. For DNA3-AgNCs and DNA4-AgNCs, the AgNCs template was “free” both in DNA3 and DNA4, but the secondary structure of LBD2 part in DNA3 and DNA4 are much different either before or after synthesis of AgNCs. As shown in Fig S7(C), the LBD2 part could form three “stem-loop” structure in DNA3 before and after synthesizing of AgNCs, and the AgNCs were very near (only five A bases away) to the third stem (the two base paired stem) when DNA3-AgNCs was formed. The LBD2 part formed only two “stem-loop” structure in DNA4 and DNA4-AgNCs, and the AgNCs was relatively far away (13 bases away) from the second stem in DNA4-AgNCs, as shown in Fig S7(D). Stem-loop structure could have important effect on the photoluminescence properties of DNA-AgNCs as reported by Gwinn.61 Therefore, the photoluminescence intensity and QY of DNA3-AgNCs were much higher than that of DNA4-AgNCs and it was the strongest among the four designed DNA-AgNCs probes. The transmission electron microscopy (TEM) images in Fig S5 showed that the average diameter of the synthesized DNA-AgNCs was about 2.5 nm except DNA2-AgNCs with a diameter larger than 2.5 nm (~ 4 nm), and maybe it can explain why the emission wavelength of DNA2-AgNCs shifted to longer wavelength than the other three.


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

The sensing ability of the four designed DNA-AgNCs to lysozyme was then conducted. Fig 4 displayed the relationship between the photoluminescence enhancement factor, (I-I0)/I0, and the lysozyme concentration ranging from 0 µM to 6 µM, where I0 and I were the photoluminescence intensity of DNA-AgNCs at 625 nm (640 nm for DNA2-AgNCs) when excited at 555 nm in the absence and presence of lysozyme respectively. It can be seen that the response sensitivity was in the order of DNA1-AgNCs > DNA2-AgNCs (for LBD1) and DNA4-AgNCs > DNA3-AgNCs (for LBD2). It was deemed that the secondary structure of LBD1 or LBD2 should be responsible for the sensing ability. For DNA1 (the secondary structure was shown in Fig S7(A)), the LBD1 part could keep a secondary structure as that of free LBD1 (shown as Fig S6(A)) both before and after synthesis of AgNCs; while for DNA2 (Fig S7(B)), the LBD1 part was not keeping a secondary structure as that of free LBD1 but could formed partially after synthesis of AgNCs. Or it can be thought the content of “effective LBD1” in DNA1-AgNCs was higher than that in DNA2-AgNCs, thus the response sensitivity was DNA1-AgNCs > DNA2-AgNCs. For DNA3AgNCs and DNA4-AgNCs, the LBD2 part could keep the secondary structure (shown as Fig S7(D)) as that of free LBD2 (Fig S6(B)) in DNA4-AgNCs; While in DNA3-AgNCs, the LBD2 part formed another more compact secondary structure (as shown in Fig S7(C)) different from the structure of “free” LBD2, which lowed the binding ability of this probe greatly, thus the response ability was DNA4-AgNCs > DNA3-AgNCs even though the photoluminescence of DNA3-AgNCs was much stronger than the others. To prove this, control experiments using other two probes in which LBD2 part had different secondary structure compared with that in DNA3 and DNA4 were done. As shown in Fig S9, the response signal of DNA6-AgNCs (compared with DNA3-AgNCs, DNA6-AgNCs had no (A)5 linker and the LBD2 part could keep a secondary structure as that of the free LBD2) was higher than that of DNA3-AgNCs; also, the


16

Page 17 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

response ability of DNA7-AgNCs (the LBD2 part of DNA7-AgNCs could not keep a secondary structure as that in DNA4-AgNCs) was much lower than that of DNA4-AgNCs, and nearly the same as that of DNA3.

Fig 4. Plots of enhancement factor, (I-I0)/I0, versus the lysozyme concentration from 0 µM to 6.0 µM of the four DNA-AgNCs. I was the photoluminescence intensity in the presence of lysozyme and I0 was the photoluminescence intensity in the absence of lysozyme. The concentration effect of AgNCs probe on the sensing ability to lysozyme was also examined. As shown in Fig S10, when the concentration of DNA1-AgNCs probe lowered from 1200 nM to 200 nM, the response to low concentration of lysozyme increased, but the response range narrowed. Thus, the sensing ability of the DNA1-AgNCs aptamer sensor could be tuned according to the target of lysozyme detected by adjusting the probe concentration. To take into account of the sensing sensitivity and the response range, 600 nM of DNA1-AgNCs was selected for the study. Fig 5(A) showed the photoluminescence spectra of 600 nM of DNA1-AgNCs in the presence of different concentration of lysozyme from 0 to 10.0 µM. When DNA1-AgNCs were excited at 555 nm, the photoluminescence intensity increased gradually in the lysozyme concentration


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

range from 0 to 8.0 µM, then decreased when the lysozyme concentration increased to 10.0 µM. Fig 5(B) (DNA4-AgNCs was in Fig S11(A)) gave the relationship between the photoluminescence enhancement factor, (I-I0)/I0, and the lysozyme concentration from 0 to 10.0 µM. Two segments of linear range could be found in the lysozyme concentration range from 0 to 8.0 µM. Fig 5(C) (DNA4-AgNCs in Fig S11(C)) presented the relationship between the photoluminescence enhancement of DNA1-AgNCs and the lysozyme concentration from 0.02 µM to 3.0 µM. The linear regression equation could be expressed as (I-I0)/I0 = 1.1007CLys (µM) + 0.3284, with correlation coefficient 0.9901 (n=6). And the relationship between the photoluminescence changes and lysozyme concentration from 3.0 µM to 8.0 µM was shown in Fig 5(D) (DNA4-AgNCs in Fig S11(E)), and the linear regression equation could be expressed as (I-I0)/I0 = 3.6979CLys (µM) – 8.1449, with correlation coefficient 0.9960 (n=6). The two linear fitting equations could be used for quantitative determination of lysozyme according to different concentration detected. The slope of the linear fitting equation in the lysozyme concentration range from 3.0 µM to 8.0 µM was higher than that in the concentration range from 0.02 µM to 3.0 µM, indicating that the response of AgNCs to higher concentration of lysozyme based on photoluminescence enhancement due to the coupling between surface plasmon and emitter was more sensitive than that for lower concentration of lysozyme due to the photoluminescence enhancement of emitter only.


18

Page 19 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig 5. (A) Photoluminescence spectra of 600 nM DNA1-AgNCs in the presence of different concentration of lysozyme from 0 µM to 10.0 µM. (The photoluminescence intensity increased with a concentration from 0, 0.02, 0.05, 0.4, 0.8, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 to 8.0 µM and decreased from 8.0 to 10.0 µM) (B) The relationship between photoluminescence enhancement factor, (I-I0)/I0, and the concentration of lysozyme. And the linear region of photoluminescence changes vs. lysozyme concentration from (C) 0.02 µM to 3.0 µM and (D) 3.0 µM to 8.0 µM. The excitation wavelength was 555 nm.


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

3.3 The mechanism of photoluminescence enhancement of DNA-AgNCs for sensing lysozyme To explore the mechanism of the photoluminescence enhancement of DNA1-AgNCs in the presence of lysozyme when excited at 555 nm, the photoluminescence spectra changes when excited at 350 nm and 435 nm were also investigated. The photoluminescence increased gradually in the lysozyme concentration ranging from 0 µM to 6.0 µM when DNA1-AgNCs were excited at 350 nm, but slight light scattering appeared when lysozyme concentration was ranged from 3.0 to 6.0 µM, as shown in Fig 6(A). And as shown in Fig 6(B), when the AgNCs were excited at 435 nm, the photoluminescence intensity of DNA1-AgNCs remained unchanged in the lysozyme concentration from 0 µM to 3.0 µM, while that increased a lot in the lysozyme concentration ranging from 3.0 µM to 6.0 µM. Therefore, the enhancement of the photoluminescence of DNA1-AgNCs when excited at 555 nm could be attributed to the emitter in the lysozyme concentration from 0 to 3.0 µM, and to the coupling between surface plasmon and the emitter in the lysozyme concentration from 3.0 µM to 6.0 µM.

Fig 6. The photoluminescence spectra of DNA1-AgNCs in the presence of different concentration of lysozyme from 0 to 6.0 µM (A) under 350 nm excitation and (B) under 435 nm excitation.


20

Page 21 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

It was estimated that the microenvironment of AgNCs changed due to the addition of lysozyme and then led to the enhancement of photoluminescence. The conformational change of DNA1-AgNCs could also be proved by circular dichroism (CD) measurement. As shown in Fig 7, the ellipticity at 220 nm became more positive while that at 272 nm became less positive with a slight redshift when the concentration of lysozyme increased from 0 µM to 3.0 µM. This indicated that the binding of LBD1 with lysozyme provided better protection for AgNCs and resistance to the environmental quenching.62 The reason of photoluminescence enhancement for adding higher concentration of lysozyme could be that the distance between AgNCs was more appropriate for the coupling between the surface plasmon and the emitter.

Fig 7. Circular dichroism (CD) spectra of DNA1-AgNCs at different concentration of lysozyme. The arrow indicated the signal changes when lysozyme was added. To testify the selectivity of the sensing platform for lysozyme, four control proteins such as BSA, trypsin, thrombin and IgG were tested. Fig 8(A) showed the photoluminescence enhancement of DNA1-AgNCs after addition of lysozyme or four other control proteins. It can be seen that only lysozyme could induce obvious photoluminescence enhancement of DNA1-


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

AgNCs. The results indicated that DNA1-AgNCs possessed selective response towards lysozyme which could be attributed to the specific binding of LBD1 to its target. To further examine the photoluminescence change of DNA1-AgNCs in the presence of lysozyme was due to the binding of LBD1 with lysozyme, control DNA-AgNCs probes were used to test the response. In control DNA-AgNCs probes, the DNA scaffold for AgNCs was the same, while the other part of the DNA sequence was not the same as that of LBD1 (partially the same as LBD1, the sequences were shown in Experimental). As shown in Fig 8(B), the photoluminescence of control DNA-AgNCs hardly changed with increasing the concentration of lysozyme, and only DNA1-AgNCs had obvious response to lysozyme.

Fig 8. (A) Response selectivity of DNA1-AgNCs towards lysozyme. The concentration of lysozyme and other four control proteins was all 3.0 µM. (B) Response of DNA1-AgNCs and two control DNA-AgNCs towards lysozyme. CONCLUSIONS In summary, this work studied the photoluminescence origin and biosensing application of DNA-AgNCs. Firstly, the photoluminescence origin of DNA-AgNCs was studied. The


22

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

photoluminescence was manifested to be from coupling between the surface plasmon and the emitter under visible light excitation above 500 nm for both red only and two components of purple and red emitting DNA-AgNCs, thus the emission wavelength was dependent on excitation wavelength. The photoluminescence was intrinsic fluorescence which was only related to the emitter for red only emitting DNA-AgNCs but a coupling between the surface plasmon and the emitter for two components of purple and red emitting DNA-AgNCs, when excited under ultraviolet light. While the photoluminescence was only related to the surface plasmon when excited from 400 nm to 500 nm for both red only and two components of purple and red emitting DNA-AgNCs. And then, four DNA probes were designed and their photoluminescence properties as well as the sensing ability for lysozyme were investigated. It was found the difference in DNA bases, sequence and secondary structure could affect the photoluminescence and sensing properties of the DNA-AgNCs probe. The sensing mechanism based on photoluminescence enhancing was also proposed. ASSOCIATED CONTENT Supporting Information: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This project was supported by the National Natural Science Foundation of China (No. 20905008), and the Fundamental Research Funds for the Central Universities (No. 2012LYB17). REFERENCES (1) Zheng, J.; Nicovich, P. R.; Dickson, R. M. Highly Fluorescent Noble-Metal Quantum Dots. Annu. Rev. Phys. Chem. 2007, 58, 409-431.


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

(2) Shang, L.; Dong, S. J. Silver nanocluster-based fluorescent sensors for sensitive detection of Cu (Ⅱ). J. Mater. Chem. 2008, 18, 4636-4640. (3) Zhang, Y. Y.; Jiang, H.; Ge, W.; Li, Q. W.; Wang, X. M. Cytidine-Directed Rapid Synthesis of Water-Soluble and Highly Yellow Fluorescent Bimetallic Au/Ag Nanoclusters. Langmuir 2014, 30, 10910-10917. (4) Vosch, T.; Antoku, Y.; Hsiang, J. C.; Richards, C. I.; Gonzalez, J. I.; Dickson, R. M. Strongly emissive individual DNA-encapsulated Ag nanoclusters as single-molecule fluorophores. Proc. Natl. Acad. Sci. U.S.A 2007, 104, 12616-12621. (5) Souza, N. D. All that glitters but does not blink. Nat. Methods 2007, 4, 540-540. (6) Yang, C. Y.; Dou, S. K.; Xiang, B. T.; Chai, Y.; Yuan, Y. Q.; Ruo, Y. In Situ DNATemplated Synthesis of silver nanoclusters for Ultrasensitive and Label-Free Electrochemical Detection of MicroRNA. ACS Appl. Mater. Interfaces 2015, 7, 1188-1193. (7) Shen, C. C.; Xia, X. D.; H. S. Q.; Yang, M. H.; Wang, J. X. Silver nanoclusters-based fluorescence assay of protein kinase activity and inhibition. Anal. Chem. 2015, 7, 1188-1193. (8) Li, J. J.; You, J.; Dai, Y.; Shi, M. L.; Han, C. P.; Xu, K. Gadolinium Oxide Nanoparticles and Aptamer-Functionalized

Silver

Nanoclusters-Based

Multimodal

Molecular

Imaging

Nanoprobe for Optical/Magnetic Resonance Cancer Cell Imaging. Anal. Chem. 2014, 86, 11306-11311. (9) Wang, Y.; Dai, C.; Yan, X. P. Fabrication of folate bioconjugated near-infared fluorescent silver nanoclusters for targeted in vitro and in vivo bioimaging. Chem. Commun. 2014, 50, 14341-14344.


24

Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(10) Gwinn, E. G.; O’Neill, P.; Guerrero, A. J.; Bouwmeester, D.; Fygenson, D. K. SequenceDependent Fluorescence of DNA-Hosted Silver Nanoclusters. Adv. Mater. 2008, 20, 279283. (11) Sengupta, B.; Ritchie, C. M.; Buckman, J. G.; Johnsen, K. R.; Goodwin, P. M.; Petty, J. P. Base-Directed Formation of Fluorescent Silver Clusters. J. Phys. Chem. C 2008, 112, 1877618782. (12) Shah, P.; Rorvig-Lund, A.; Chaabane, S. B.; Thulstrup, P. W.; Kjaergaard, H. G.; Fron, E.; Hofkens, J.; Yang, S. W.; T. Vosch. Design Aspects of Bright Red Emissive Silver Nanoclusters/DNA Probes for MicroDNA Detection. ACS Nano 2012, 6, 8803-8814. (13) Richards, C. I.; Choi, S.; Hsiang, J. C.; Antoku, Y.; Vosch, T.; Bongiorno, A.; Tzeng, Y. L.; Dickson, R. M. Olignucleotide-Stabilized Ag Nanocluster Fluorophores. J. Am. Chem. Soc. 2008, 130, 5038-5039. (14) Sharma, J.; Yeh, H. C.; Yoo, H.; Werner, J. H.; Martinez, J. S. A complementary palette of fluorescent silver nanoclusters. Chem. Commun. 2010, 46, 3280-3282. (15) O’Neill, P. R.; Velazquez, L. R.; Dunn, D. G.; Gwinn, E. G.; Fygenson, D. K. Hairpins with Poly-C Loops Stabilize Four Types of Fluorescent Agn : DNA. J. Phys. Chem. C 2009, 113, 4229-4233. (16) Zhou, Z. X.; Du, Y.; Zhang, L. B.; Dong, S. J. A label-free, G-quadruplex DNAzyme-based fluorescent probe for signal-amplified DNA detection and turn-on assay of endonuclease. Biosens. Bioelectron. 2012, 34, 100-105. (17) Ma, K.; Shao, Y.; Cui, Q. H.; Wu, F.; Xu, S. J.; Liu, G. Y. Base-Stacking-Determined Fluorescence Emission of DNA Abasic Site-Templated Silver Nanoclusters. Langmuir 2012, 28, 15313-15322.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

(18) Cathcart, N.; Mistry, P.; Makra, C.; Pietrobon, B.; Coombs, N.; Niaraki, M. J.; Kitaev, V. Chiral Thiol-Stabilized Silver Nanoclusters with Well-Resolved Optical Transitions Synthesized by a Facile Etching Procedure in Aqueous Solutions. Langmuir 2009, 25, 58405846. (19) Ganguly, M.; Pal, A.; Negishi, Y.; Pal. T. Synthesis of Highly Fluorescent Silver Clusters on Gold(I) Surface. Langmuir 2013, 29, 2033-2043. (20) Cliffel, D. E.; Zamborini, F. P.; Gross, S. M.; Murray, R. W. MercaptoammoniumMonolayer-Protected, Water-Soluble Gold, Silver, and Palladium Clusters. Langmuir 2000, 16, 9699-9702. (21) Yanagihara, N.; Uchida, K.; Wakabayashi, M.; Uetake, Y.; Hara, T. Effect of Radical Initiators on the Size and Formation of Silver Nanoclusters in Poly(methyl methacrylate). Langmuir 1999, 15, 3038-3041. (22) Qu, F.; Li, N. B.; Luo, H. Q. Highly Sensitive Fluorescent and Colorimetric pH Sensor Based on Polyethylenimine-Capped Silver Nanoclusters. Langmuir 2013, 29, 1199-1205. (23) Zhou, T. Y.; Huang, Y. H.; Li, W. B.; Cai, Z.M.; Luo, F.; Yang, C. L.; Chen, X. Facile synthesis of red-emitting lysozyme-stabilized Ag nanoclusters. Nanoscale 2012, 4, 53125315. (24) Guevel, X. L.; Hotzer, B.; Jung, G.; Hollemeyer, K.; Trouillet, V.; Schneider, M. Formation of Fluorescent Metal (Au, Ag) Nanoclusters Capped in Bovine Serum Albumin Followed by Fluorescent and Spectroscopy. J. Phys. Chem. C 2011, 115, 10955-10963. (25) Mathew, A.; Sajanlal, P. R.; Pradeep, T. A fifteen atom silver cluster confined in bovine serum albumin. J. Mater. Chem. 2011, 21, 11205-11212.


26

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(26) Eichhom, G. L. Complexes of nucleosides and nucleotides. Inorg. Biochem. 1973, 2, 11911209. (27) Lorenzo, B.; Glenn A, B. Nucleic acids and nucleotide-mediated synthesis of inorganic nanoparticles. Nat Nanotechnol. 2008, 3, 81-87. (28) Petty, J. T.; Zheng, J.; Hud, N. V.; Dickson, R. M. DNA-Templated Ag Nanocluster Formation. J. Am. Chem. Soc. 2004, 126, 5207-5212. (29) Diez, I.; Ras, R. H. A.; Kanyuk, M. I.; Demchenko, A. P. On heterogeneity in fluorescent few-atom silver nanoclusters. Phys. Chem. Chem. Phys. 2013, 15, 979-985. (30) Ritchie, C. M.; Johnsen, K. R.; Kiser, J, R.; Antoku, Y.; Dickson, R.M.; Petty, J. T. Ag Nanocluster Formation Using a Cytosine Oligonucleotide Template. J. Phys. Chem. C Nanomater Interfaces. 2007, 111, 175-181. (31) Shang, L.; Dong, S. J. Facile preparation of water-soluble fluorescent silver nanoclusters using a polyelectrolyte template. Chem. Commun. 2008, 1088-1090. (32) Shen, Z.; Duan, H.; Frey, H. Water-Soluble Fluorescent Ag Nanoclusters obtained from Multiarm Star Poly (acrylic acid) as “Molecular Hydrogel” Templates. Adv. Mater. 2007, 19, 349-352. (33) Morishita, K.; MacLean, J. L.; Liu, B.; Jiang, H.; Liu, J. Correlation of photobleaching, oxidation and metal induced fluorescence quenching of DNA-templated silver nanoclusters. Nanoscale 2013, 5, 2840. (34) Chen, Y. T.; Yang, T. Q.; Pan, H, F.; Yuan, Y. F.; Chen, L.; Liu, M. W.; Zhang K.; Zhang, S. J.; Wu, P.; Xu, J. H. Photoemission Mechanism of Water-Soluble Silver Nanoclusters: Ligand-to-Metal-Metal Charge Transfer vs Strong Coupling between Surface Plasmon and Emitters. J. Am. Chem. Soc. 2014, 136, 1686-1689.


27

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

(35) Kirby, R.; Cho, E. J.; Gehrke, B.; Bayer, T.; Park, Y. S.; Neikirk, D. P.; McDevitt, J. T.; Ellington, A. D. Aptamer-Based Sensor Arrays for the Detection and Quantitation of Proteins. Anal. Chem. 2004, 76, 4066-4075. (36) Kawde, A. N.; Rodriguez, M. C.; Lee, T. M. H.; Wang, J. Label-free bioelectronic detection of aptamer-protein interactions. Electrochem. Commun. 2005, 7, 537-540. (37) Rohrbach, F.; Karadeniz, H.; Erdem, A.; Famulok, M.; Mayer, G. Label-free impedimetric aptasensor for lysozyme detection based on carbon nanotube-modified screen-printed electrodes. Anal. Biochem. 2012, 421, 454-459. (38) Cheng, A. K. H.; Ge, B. X.; Yu, Z. H. Aptamer-Based Biosensors for Label-Free Voltammetric Detection of Lysozyme. Anal. Chem. 2007, 79, 5158-5164. (39) Huang, H. P.; Jie, G. F.; Cui, R. J.; Zhu, J. J. DNA aptamer-based detection of lysozyme by an electrochemiluminescence assay coupled to quantum dots. Electrochem. Commun. 2009, 11, 816-818. (40) Tran, D. T.; Janssen, K. P. F.; Pollet, J.; Lammertyn, E.; Anne, J.; Schepdael, A. V.; Lammertyn, J. Selection and Characterization of DNA Aptamers for Egg White Lysozyme. Molecules 2010, 15, 1127-1140. (41) Truong, P. L.; Choi, S. P.; Sim, S. J. Amplification of Resonant Rayleigh Light Scattering Response Using Immunogold Colloids for Detection of Lysozyme. Small 2013, 9, 34853492. (42) Wang, X. Y.; Xu, Y.; Chen, Y.; Li, L. M.; Liu, F.; Li, Na. The gold-nanoparticle-based surface plasmon resonance light scattering and visual DNA aptasensor for lysozyme. Anal. Bioanal. Chem. 2011, 400, 2085-2091.


28

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(43) Zou, M. J.; Chen, Y.; Xu, X.; Huang, H. D.; Liu, F.; Li, N. The homogeneous fluorescence anisotropic sensing of salivary lysozyme using the 6-carboxyfluorescein-labeled DNA aptamer. Biosens. Bioelectron. 2012, 32, 148-154. (44) Han, B.; Zhao, C.; Yin, J. F.; Wang, H. L. High performance aptamer affinity chromatography for single-step selective extraction and screening of basic protein lysozyme. J. Chromatogr. B 2012, 903, 112-117. (45) Zheng, J.; Ding, Y.; Tian, B. Z.; Wang, Z. L.; Zhuang, X. W. Luminescent and Raman Active Silver Nanoparticles with polycrystalline Structure. J. Am. Chem. Soc. 2008, 130, 10472-10473. (46) Pal. S.; Varghese, R.; Deng, Z. T.; Zhao, Z.; Kumar, A.; Yan, H.; Liu, Y. Site-Specific Synthesis and In Situ Immobilization of Fluorescent Silver Nanoclusters on DNA Nanoscaffolds by Use of the Tollens Reaction. Angew. Chem., Int. Ed. 2011, 50, 4176-4179. (47) Wu, Z. K.; Jin, R. C. On the Ligand’s Role in the Fluorescence of Gold Nanoclusters. Nano Lett. 2010, 10, 2568-2573. (48) Luo, Z. T.; Yuan, X.; Yu, Y.; Zhang, Q. B.; Leong, D. T.; Lee, J. Y.; Xie, J. P. From Aggregation-Induced Emission of Au(I)-Thiolate Complexes to Ultrabright Au(0)@Au(I)Thiolate Core-Shell Nanoclusters. J. Am. Chem. Soc. 2012, 134, 16662. (49) Pelton, M.; Tang, Y.; Bakr. O. M.; Stellacci, F. Long-Lived Charge-Separated States in Ligand-Stabilized Silver Clusters. J. Am. Chem. Soc. 2012, 134, 11856-11859. (50) Huang, Z. Z.; Pu, F.; Lin, Y. H.; Ren, J. S.; Qu, X. G. Modulating DNA-templated silver nanoclusters for fluorescence turn-on detection of thiol compounds. Chem. Commun. 2011, 47, 3487–3489.


29

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

(51) Shaviv, E.; Schubert, O.; Santos, M. A.; Goldoni, G.; Felice, R. D.; Vallée, F.; Fatti, N. D.; Banin, U.; Sonnichsen, C. Absoption Properties of Metal-Semiconductor Hybrid Nanoparticles. ACS Nano 2011, 5, 4712-4719. (52) Stephan, L.; Mostafa A, E. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nano-dots and Nano-rods. J. Phys. Chem. B 1999, 103, 8410-8426. (53) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668-677. (54) Guan, Z. P.; Li, S.; Cheng, P. B. S.; Zhou, N.; Gao, N. Y.; Xu, Q. H. Band-Selective Coupling-Induced Enhancement of Two-Photon Photoluminescence in Gold Nanocubes and Its Application as Turn-on Fluorescent Probes for Cysteine and Glutathione. Appl. Mater. Interfaces 2012, 4, 5711-5716. (55) Amendola, V.; Bakr, O. M.; Stellacci, F. A Study of the Surface Plasmon Resonance of Silver Nanoparticles by the Discrete Dipole Approximation Method: Effect of Shape, Size, Structure, and Assembly. Plasmonics 2010, 5, 85-97. (56) Sillanpää, M.; Lahtine, M.; Dubey, S. P. Tansy fruit mediated greener synthesis of silver and gold nanoparticles. Process Biochem. 2010, 45, 1065-1071. (57) Zhang, M.; Liu, Y. Q.; Ye, B. C. Colorimetric assay for parallel detection of Cd2+, Ni2+ and Co2+ using peptide-modified gold nanoparticles. Analyst 2012, 137, 601. (58) Bellessa, J.; Bonnand, C.; Plenet, J. C. Strong Coupling between Surface Plasmons and Excitons in an Organic Semiconductor. J. Phys. Rev. Lett. 2004, 93, 036404-3.


30

Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(59) Salomon, A.; Gordon, R. J.; Prior, Y.; Seideman, T.; Sukharev, M. Strong Coupling between Molecular Excited States and Surface Plasmon Modes of a Slit Array in Thin Metal Film. Phys. Rev. Lett. 2012, 109, 073002-1. (60) Gomez, D. E.; Vernon, K. C.; Mulvaney, P.; Davis, T. J. Surface Plasmon Mediated Strong Exciton-Photon Coupling in Semiconduction. Nano Lett. 2010, 10, 274-278. (61) Gwinn, E. G.; O’Neill, P.; Guerrero, A. J.; Bouwmeester, D.; Fygenson, D. K. SequenceDependent Fluorescence of DNA-Hosted Silver Nanoclusters. Adv. Mater. 2008, 20, 279-283. (62) Lan, G. Y.; Huang, C. C.; Chang, H. T. Silver nanoclusters as fluorescent probes for selective and sensitive detection of copper ions. Chem. Commun. 2010, 46, 1257-1259.


31

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

Table of Contents


32

Photoluminescence Mechanism of DNA ... - ACS Publications

Recommend Documents