Abstract
In this thesis two different systems are investigated envisioning their potential applications:
DNA-templated silver nanoclusters (DNA-AgNCs) and ionic self-assembled (ISA)
nanostructures based on azo-dyes. Mainly Visible-NIR spectroscopy was used to probe
electronic transitions with absorbance and fluorescence techniques.
Fluorophores are an essential tool for biology, as they are used to observe biological processes in
living cells in fluorescence microscopy. The search for new fluorophores with better
characteristics is further motivated by the recent development of super-resolution microscopy.
Many other applications are possible such as detection of analytes, pH detection or their use as
active layer of Organic Light Emitting Diodes (OLEDs). The fluorophores studied here, DNAAgNCs,
are few nanometer sized and formed by a few to ca. 20 silver atoms templated by one or
two single stranded DNA (ssDNA) scaffolds. By choosing different DNA sequences their
emission can be tuned in the visible-NIR range. Their small size, brightness, photostability and
good biocompatibility makes them a promising alternative to other commercially available
fluorophores such as organic dyes or quantum dots.
Upon synthesis of the DNA-AgNCs, many emissive species can be formed. In particular, the
single-stranded DNA (ssDNA) sequence containing 24 cytosines (C24) stabilizes a range of
emitters. Here the spectral heterogeneity and synthesis reproducibility of C24-AgNCs is
spectroscopically characterized from the average fluorescence decay time as a function of
emission wavelength (Average decay time spectra). This proved to be a robust method to
characterize the complexity of the system due to the multi-exponential decay of the emitters.
In order to study the electroluminescence from the C24-AgNCs in Self-Assembled Monolayer
(SAM), C24-AgNCs with an attached Thiol (-SH) group to the 3’ end of the DNA strand (C24-
(C6)3Thio-AgNCs) were linked to a gold substrate. Since quenching from the gold surface
impeded the observation of the molecules by fluorescence, other techniques such as Atomic
Force Microscopy (AFM), conductive AFM and X-ray photoelectron spectroscopy were used to
confirm the presence of the C24-(C6)3Thio-AgNCs. The conductive AFM measurements
indicated that the clusters behave as molecular switches. Larger scale electroluminescence was
also attempted, and DNA-AgNCs stabilized by a different DNA sequence (“Red-AgNCs”) were
tried as an active material of an OLED. Due to the low homogeneity of the active layer, no
emission was observed the OLEDs. For this reason, work was put in optimizing the homogeneity
of a DNA-AgNC thin film, bringing the DNA-AgNC-OLED closer to its realization.
The Ag local structure in IR emitting DNA-AgNCs (IR-AgNCs) stabilized by the
CCCACCCACCCTCCCA sequence was probed with Ag K-Edge Extended X-Ray Absorption
Fine Structure (Ag K-Edge EXAFS). The mixture of the DNA sequence and silver ions was also
measured prior to reduction, enabling a comparison of the Ag local structure of the DNA-AgNCs
before and after reduction. Two reduced samples of IR-AgNCs were measured, a sample purified
by High Performance Liquid Chromatography (HPLC) and a non-purified sample. The samples
were characterized spectroscopically to support the EXAFS measurements.
C24-AgNCs samples were prepared for single-molecule spectral, time-resolved and polarization
studies that contributed to the knowledge on this sequence. Purified IR-AgNCs were prepared
and 2 Dimensional Electronic Spectroscopy (2DES) was performed by our collaborators at the
University of Lund.
Self-assembled nanostructures are interesting due to their reduced cost, high achievable degree
of supramolecular order, and potential application in molecular electronics. The degree of order
is related to the performance of molecular devices. Ionic Self-Assembled (ISA) materials formed
by surfactant chains and azo-dyes featuring long range order can be easily synthesized, and can
form lamellar structures after being spin cast on a transparent substrate. Here, using polarized
absorption spectroscopy, the order of such materials was characterized. The excitation
photoselectivity of molecules enabled probing the absorption transition moment of the azo-dyes.
An absorption band in the visible spectrum was measured using the tilted plate method and the
stretched polymer-film method. Adapting the theory from the literature and using computational
tools, the average angle between absorption transition moment of the azo dyes and the normal of
the substrate was calculated. This angle corresponds to two possible angles of the long axis of
the azo dyes respect to the normal of the thin film substrate. The presence of supramolecular
order in the sample was deduced from the angle between the transition moment and the normal
of the substrate.
DNA-templated silver nanoclusters (DNA-AgNCs) and ionic self-assembled (ISA)
nanostructures based on azo-dyes. Mainly Visible-NIR spectroscopy was used to probe
electronic transitions with absorbance and fluorescence techniques.
Fluorophores are an essential tool for biology, as they are used to observe biological processes in
living cells in fluorescence microscopy. The search for new fluorophores with better
characteristics is further motivated by the recent development of super-resolution microscopy.
Many other applications are possible such as detection of analytes, pH detection or their use as
active layer of Organic Light Emitting Diodes (OLEDs). The fluorophores studied here, DNAAgNCs,
are few nanometer sized and formed by a few to ca. 20 silver atoms templated by one or
two single stranded DNA (ssDNA) scaffolds. By choosing different DNA sequences their
emission can be tuned in the visible-NIR range. Their small size, brightness, photostability and
good biocompatibility makes them a promising alternative to other commercially available
fluorophores such as organic dyes or quantum dots.
Upon synthesis of the DNA-AgNCs, many emissive species can be formed. In particular, the
single-stranded DNA (ssDNA) sequence containing 24 cytosines (C24) stabilizes a range of
emitters. Here the spectral heterogeneity and synthesis reproducibility of C24-AgNCs is
spectroscopically characterized from the average fluorescence decay time as a function of
emission wavelength (Average decay time spectra). This proved to be a robust method to
characterize the complexity of the system due to the multi-exponential decay of the emitters.
In order to study the electroluminescence from the C24-AgNCs in Self-Assembled Monolayer
(SAM), C24-AgNCs with an attached Thiol (-SH) group to the 3’ end of the DNA strand (C24-
(C6)3Thio-AgNCs) were linked to a gold substrate. Since quenching from the gold surface
impeded the observation of the molecules by fluorescence, other techniques such as Atomic
Force Microscopy (AFM), conductive AFM and X-ray photoelectron spectroscopy were used to
confirm the presence of the C24-(C6)3Thio-AgNCs. The conductive AFM measurements
indicated that the clusters behave as molecular switches. Larger scale electroluminescence was
also attempted, and DNA-AgNCs stabilized by a different DNA sequence (“Red-AgNCs”) were
tried as an active material of an OLED. Due to the low homogeneity of the active layer, no
emission was observed the OLEDs. For this reason, work was put in optimizing the homogeneity
of a DNA-AgNC thin film, bringing the DNA-AgNC-OLED closer to its realization.
The Ag local structure in IR emitting DNA-AgNCs (IR-AgNCs) stabilized by the
CCCACCCACCCTCCCA sequence was probed with Ag K-Edge Extended X-Ray Absorption
Fine Structure (Ag K-Edge EXAFS). The mixture of the DNA sequence and silver ions was also
measured prior to reduction, enabling a comparison of the Ag local structure of the DNA-AgNCs
before and after reduction. Two reduced samples of IR-AgNCs were measured, a sample purified
by High Performance Liquid Chromatography (HPLC) and a non-purified sample. The samples
were characterized spectroscopically to support the EXAFS measurements.
C24-AgNCs samples were prepared for single-molecule spectral, time-resolved and polarization
studies that contributed to the knowledge on this sequence. Purified IR-AgNCs were prepared
and 2 Dimensional Electronic Spectroscopy (2DES) was performed by our collaborators at the
University of Lund.
Self-assembled nanostructures are interesting due to their reduced cost, high achievable degree
of supramolecular order, and potential application in molecular electronics. The degree of order
is related to the performance of molecular devices. Ionic Self-Assembled (ISA) materials formed
by surfactant chains and azo-dyes featuring long range order can be easily synthesized, and can
form lamellar structures after being spin cast on a transparent substrate. Here, using polarized
absorption spectroscopy, the order of such materials was characterized. The excitation
photoselectivity of molecules enabled probing the absorption transition moment of the azo-dyes.
An absorption band in the visible spectrum was measured using the tilted plate method and the
stretched polymer-film method. Adapting the theory from the literature and using computational
tools, the average angle between absorption transition moment of the azo dyes and the normal of
the substrate was calculated. This angle corresponds to two possible angles of the long axis of
the azo dyes respect to the normal of the thin film substrate. The presence of supramolecular
order in the sample was deduced from the angle between the transition moment and the normal
of the substrate.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2016 |