Chris D Geddes

University of Maryland, Baltimore County, Baltimore, Maryland, United States

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Publications (273)750.35 Total impact

  • Jan Karolin, Chris D. Geddes
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    ABSTRACT: We report spectroscopic properties of a new nanoparticle sizing fluorophore and demonstrate its applicability to characterize silica particles in colloidal solutions using steady-state anisotropy measurements and Perrin graph analysis. The strong electrostatic interaction between the cationic particle sizing fluorophore and the SiO2 surface is demonstrated by comparing data to that recorded for the anionic fluorophore fluorescein. The Perrin analysis reports a mean particle radius of 6.35 nm, which is larger than the manufacture specified radius of 3.5 nm, possibly indicating a degree of aggregation of the particles. The anionic fluorophore fluorescein shows no interaction with the SiO2 surface, and reports a hydrodynamic radius of 0.40 nm. We thus speculate that fluorescein can be used as an internal microviscosity probe for colloidal silica systems. The intensity averaged fluorescence lifetime of the particle sizing fluorophore and fluorescein are approximately 16 ns and 4 ns, respectively.
    Dyes and Pigments 01/2015; 112:50–53. · 3.47 Impact Factor
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    Anatoliy Dragan, Chris D Geddes
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    ABSTRACT: We present a potentially highly sensitive and selective bio-assay for the potential detection of any five different DNA sequences from one sample in one well. The assay is based on a DNA "rapid catch and signal" (DNA-RCS) technology developed for the detection of different DNA sequences from a sample well area. Our signal amplification utilizes the metal-enhanced fluorescence (MEF) of dyes attached to the probe-DNAs, which hybridizes with the pre-formed mixture of anchor-DNA scaffolds on silver island films (SiFs). Low-power microwave irradiation accelerates both the formation of the anchor-DNA scaffold on the SiF-surface and anchor/probe DNA hybridization, i.e. "rapid catch" of target DNAs from a bulk solution, decreasing the assay run time from hours to only a few seconds. Localization of signaling dye-labels close to the SiFs make them extremely photostable, which allows for collecting/integrating the signal over a long time period. To demonstrate a 5 color DNA assay (5-plex) we have used a range of readily available Alexa™ dyes. Advantages and perspectives of the RCS-technologies ability to detect 5 different DNA sequences from within one plate-well are discussed.
    Journal of Fluorescence 09/2014; · 1.79 Impact Factor
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    ABSTRACT: Clostridium difficile is the primary cause of antibiotic associated diarrhea in humans and is a significant cause of morbidity and mortality. Thus the rapid and accurate identification of this pathogen in clinical samples, such as feces, is a key step in reducing the devastating impact of this disease. The bacterium produces two toxins, A and B, which are thought to be responsible for the majority of the pathology associated with the disease, although the relative contribution of each is currently a subject of debate. For this reason we have developed a rapid detection assay based on microwave-accelerated metal-enhanced fluorescence which is capable of detecting the presence of 10 bacteria in unprocessed human feces within 40 seconds. These promising results suggest that this prototype biosensor has the potential to be developed into a rapid, point of care, real time diagnostic assay for C. difficile
    PLoS ONE 08/2014; 9(8):e104334. · 3.53 Impact Factor
  • Jan Karolin, Chris Geddes
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    ABSTRACT: We report a 2 nm red shift in the fluorescence spectra observed for Rhodamine 800 dissolved in glycerol on copper substrates as compared to glass reference samples, suggesting a wavelength dependence of metal enhanced fluorescence. The full width half maximum of the blue-red spectra is about 1 nm narrower as compared to the reference sample. We speculate that the observation correlates with a specific interaction mechanism between the Rhodamine 800 transition dipole, the enhanced electric field, and subsequent plasmon coupling, an observation not yet reported.
    Applied Physics Letters 08/2014; 105(6):063102-063102-3. · 3.52 Impact Factor
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    Anatoliy Dragan, August E Graham, Chris D Geddes
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    ABSTRACT: We introduce two new fluorescent viscosity probes, SYBR Green (SG) and PicoGreen (PG), that we have studied over a broad range of viscosity and in collagen solutions. In water, both dyes have low quantum yields and excited state lifetimes, while in viscous solvents or in complex with DNA both parameters dramatically (300-1000-fold) increase. We show that in log-log scale the dependence of the dyes' quantum yield vs. viscosity is linear, the slope of which is sensitive to temperature. Application of SG and PG, as a fluorescence-based broad dynamic range viscosity probes, to the life sciences is discussed.
    Journal of Fluorescence 11/2013; · 1.79 Impact Factor
  • Chris D. Geddes
    Physical Chemistry Chemical Physics 10/2013; 15(45). · 4.20 Impact Factor
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    ABSTRACT: Distance dependent singlet and triplet metal-enhanced emission of eosin from silica coated silver island films (SiFs) has been studied by steady-state and time resolved fluorescence techniques, along with theoretical finite difference time domain (FDTD) numerical simulations, to understand how the thickness of the dielectric coating surrounding silver nanoparticles fundamentally affects luminescence enhancement. Our findings suggest that the distance dependence of metal-enhanced phenomena such as fluorescence, phosphorescence and delayed fluorescence is underpinned by the decay of the electric near-field, and depending on the actual silver silica sample embodiment, one can see either decreased or enhanced luminescence. These results not only expand our current MEF thinking but also suggest that one may well be able to approximate plasmon-enhanced luminescence values.
    Physical Chemistry Chemical Physics 10/2013; · 4.20 Impact Factor
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    ABSTRACT: A new and perspective addition to traditional fluorescent probes is the Au clusters (8–25 atoms) which can label proteins, rendering them extremely bright and photostable. In this paper, we show that albumins can quickly and effectively be labeled using microwave acceleration, which shortens the time of Au labeling from several hours to <30 s. Chromatography of Au proteins and FLIM (fluorescence lifetime imaging microscopy) reveals that Au clusters readily form and remain associated with the proteins. Subsequently, luminescence of the Au proteins (BSA, biotinylated-BSA, HSA) was studied using 3D-emission spectroscopy, time-resolved spectroscopy, and FLIM. We show that the red luminescence of the 25-atom gold cluster attached to the proteins has a broad range of emission lifetimes: about 95% of the total emission has a lifetime component ranging from 0.4 to 105 ns, and 5% is a delayed (alpha) emission with a range of lifetimes from 1 to 280 μs. The spectrum of Au delayed emission coincides with its fluorescence spectrum, suggesting that the Au delayed emission is actually delayed fluorescence (possibly, classical α-type), and emitting from the same electronic state. Our findings for the Au proteins suggest their broad applicability as new long-lived luminescence probes for the life sciences.
    The Journal of Physical Chemistry C 08/2013; 117(32):16650–16657. · 4.84 Impact Factor
  • Jan O. Karolin, Chris D Geddes
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    ABSTRACT: In this contribution we show that the Metal-Enhanced Fluorescence (MEF) Excitation Volumetric Effect (EVE), has a profound effect on the formation of Reactive Oxygen Species (ROS), such as singlet oxygen ((1)O2) and superoxide anion radical (O2(-)*), when sensitizers are placed in close proximity to plasmon supporting nanoparticulate substrates. In particular, when the singlet oxygen sensitizer rose bengal is placed on a SiFs surface, i.e. on a silver island film, the (1)O2 response to power is non-linear, and at 100 mW excitation power (535 nm) it is about 5 times higher, as compared to glass control samples, measured with the commercially available (1)O2 probe Sensor Green™. We also report a similar power dependence of superoxide generation for acridine on SiFs surfaces, but using the dihydroethidium O2(-)* probe (DHE). Our findings are consistent with our previously postulated Metal-Enhanced Fluorescence (MEF) and EVE models.
    Physical Chemistry Chemical Physics 07/2013; · 4.20 Impact Factor
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    ABSTRACT: Accurate point-of-care (POC) diagnostic tests for Chlamydia trachomatis (CT) are urgently needed for the rapid treatment of patients. In a blinded comparative study, we evaluated Microwave-Accelerated Metal-Enhanced Fluorescence (MAMEF) assays for ultra-fast and sensitive detection of CT DNA from vaginal swabs. The results of two distinct MAMEF assays were compared to nucleic acid amplification tests (NAATs). The first assay targeted the CT 16S rRNA gene and the second assay targeted the CT cryptic plasmid. Using pure CT, the MAMEF assays detected as few as 10 IFU/mL of CT in less than 9 minutes, including DNA extraction and detection. A total of 257 dry vaginal swabs from 245 adolescent women (ages 14-22) were analyzed. Swabs were eluted in water, the solutions lysed to release and fragment genomic DNA, followed by MAMEF-based DNA detection. The prevalence of CT by NAAT was 17.5%. Of the 45 NAAT CT-positive samples and 212 CT-negative samples, 33/45 and 197/212 were correctly identified by both MAMEF assays if both assays were required to be positive (sensitivity 73.3%, specificity 92.9%). Using the plasmid-based assay alone, 37/45 CT+ and 197/212 CT- were detected (sensitivity 82.2%; specificity 92.9%). Using the 16S rRNA assay alone, 34/45 CT+ and 197/212 CT- were detected (sensitivity 75.5%; specificity 92.9%). The overall % agreement with NAAT for the individual 16S rRNA and cryptic plasmid were 89.5% and 91.0%, respectively. Given the sensitivity, specificity, and rapid detection time of the plasmid-based assay, the MAMEF plasmid-based assay appears to be suited for clinical POC testing.
    Journal of clinical microbiology 06/2013; · 4.23 Impact Factor
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    C D Geddes
    Journal of Fluorescence 06/2013; 23(4). · 1.79 Impact Factor
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    Anatoliy I. Dragan, Buddha Mali, Chris D. Geddes
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    ABSTRACT: The fluorescence spectrum of Au-clusters (8- and 25-atom), which covers the spectral range 350–900 nm, is dramatically enhanced in the presence of plasmon supporting plate-well deposited nanoparticles. The wavelength-dependent metal-enhanced fluorescence (MEF spectrum) correlates well with the plasmon specific scattering spectrum, i.e. the synchronous scatter spectrum of the silver surface of plate wells. Our findings suggest that the synchronous scatter spectra of plasmon enhancing substrates is a good indicator of both the magnitude and the wavelength-dependence of MEF.
    Chemical Physics Letters 01/2013; 556:168–172. · 1.99 Impact Factor
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    ABSTRACT: In recent years both the mechanism and applications of metal-enhanced fluorescence (MEF) have attracted significant attention, yet many fundamental aspects of MEF remain unanswered or addressed. In this study, we address a fundamental aspect of MEF. Using fluorescein-labeled different length DNA scaffolds, covalently bound to silver nanodeposits, we have experimentally measured the distance dependence of the MEF effect. The enhanced fluorescence signatures, i.e., MEF, follow quite closely the theoretical decay of the near-field of the nanoparticles, calculated using finite difference time domain approaches. This implies that the mechanisms of MEF are partially underpinned by the magnitude and distribution of the electric field around near-field nanoparticles.
    Plasmonics 12/2012; 7(4). · 2.74 Impact Factor
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    ABSTRACT: In this paper, we have explored metal-enhanced fluorescence (MEF) of the Human serum albumin indicators: Albumin Blue 580, Merocyanine 540 and Bromophenol Blue in close proximity to silver nano-particles, SiFs, from both buffered and clinical samples. The photostability of the Albumin Blue 580 is shown to be much more prolonged from the SiFs as compared to glass (a control sample), potentially allowing for longer detection times to further improve assay statistics. Our findings suggest the widespread use of nanoparticulate SiFs surfaces for the enhanced detection of HSA, particularly for Hypoproteinemia, where an enhanced assay performance at low protein abundance is required.
    Journal of Fluorescence 10/2012; · 1.79 Impact Factor
  • Jan O Karolin, Chris D Geddes
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    ABSTRACT: We describe a fundamental observation in Metal-Enhanced Fluorescence (MEF), which has become a leading technology in the life sciences today, namely, how the lifetime of fluorophores near-to metallic plasmon-supporting silver islands/nanoparticles, modulates as a function of excitation power irradiance. This finding is in stark contrast to that observed in classical far-field fluorescence spectroscopy, where excitation power does not influence fluorophore radiative decay/lifetime.
    Journal of Fluorescence 10/2012; 22(6):1659-62. · 1.79 Impact Factor
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    ABSTRACT: In this study, we have investigated the fluorescence properties of SYBR Green I (SG) dye and its interaction with double-stranded DNA (dsDNA). SG/dsDNA complexes were studied using various spectroscopic techniques, including fluorescence resonance energy transfer and time-resolved fluorescence techniques. It is shown that SG quenching in the free state has an intrinsic intramolecular origin; thus, the observed >1,000-fold SG fluorescence enhancement in complex with DNA can be explained by a dampening of its intra-molecular motions. Analysis of the obtained SG/DNA binding isotherms in solutions of different ionic strength and of SG/DNA association in the presence of a DNA minor groove binder, Hoechst 33258, revealed multiple modes of interaction of SG inner groups with DNA. In addition to interaction within the DNA minor groove, both intercalation between base pairs and stabilization of the electrostatic SG/DNA complex contributed to increased SG affinity to double-stranded DNA. We show that both fluorescence and the excited state lifetime of SG dramatically increase in viscous solvents, demonstrating an approximate 200-fold enhancement in 100 % glycerol, compared to water, which also makes SG a prospective fluorescent viscosity probe. A proposed structural model of the SG/DNA complex is compared and discussed with results recently reported for the closely related PicoGreen chromophore.
    Journal of Fluorescence 04/2012; 22(4):1189-99. · 1.79 Impact Factor
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    ABSTRACT: We previously reported that neuraminidase (NA) pretreatment of human PBMCs markedly increased their cytokine response to lipopolysaccharide (LPS). To study the mechanisms by which this occurs, we transfected HEK293T cells with plasmids encoding TLR4, CD14, and MD2 (three components of the LPS receptor complex), as well as a NFκB luciferase reporting system. Both TLR4 and MD2 encoded by the plasmids are α-2,6 sialylated. HEK293T cells transfected with TLR4/MD2/CD14 responded robustly to the addition of LPS; however, omission of the MD2 plasmid abrogated this response. Addition of culture supernatants from MD2 (sMD2)-transfected HEK293T cells, but not recombinant, non-glycosylated MD2 reconstituted this response. NA treatment of sMD2 enhanced the LPS response as did NA treatment of the TLR4/CD14-transfected cell supplemented with untreated sMD2, but optimal LPS-initiated responses were observed with NA-treated TLR4/CD14-transfected cells supplemented with NA-treated sMD2. We hypothesized that removal of negatively charged sialyl residues from glycans on the TLR4 complex would hasten the dimerization of TLR4 monomers required for signaling. Co-transfection of HEK293T cells with separate plasmids encoding either YFP- or FLAG-tagged TLR4, followed by treatment with NA and stimulation with LPS, led to an earlier and more robust time-dependent dimerization of TLR4 monomers on co-immunoprecipitation, compared to untreated cells. These findings were confirmed by fluorescence resonance energy transfer (FRET) analysis. Overexpression of human Neu1 increased LPS-initiated TLR4-mediated NFκB activation and a NA inhibitor suppressed its activation. We conclude that (1) sialyl residues on TLR4 modulate LPS responsiveness, perhaps by facilitating clustering of the homodimers, and that (2) sialic acid, and perhaps other glycosyl species, regulate MD2 activity required for LPS-mediated signaling. We speculate that endogenous sialidase activity mobilized during cell activation may play a role in this regulation.
    PLoS ONE 04/2012; 7(4):e32359. · 3.53 Impact Factor
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    ABSTRACT: Rapid presymptomatic diagnosis of Bacillus anthracis at early stages of infection plays a crucial role in prompt medical intervention to prevent rapid disease progression and accumulation of lethal levels of toxin. To detect low levels of the anthrax protective antigen (PA) exotoxin in biological fluids, we have developed a metal-enhanced fluorescence (MEF)-PA assay using a combination of the MEF effect and microwave-accelerated PA protein surface absorption. The assay is based on a modified version of our "rapid catch and signal" (RCS) technology previously designed for the ultra-fast and sensitive analysis of genomic DNA sequences. Technologically, the proposed MEF-PA assay uses standard 96-well plastic plates modified with silver island films (SiFs) grown within the wells. It is shown that the fluorescent probe, covalently attached to the secondary antibody, plays a crucial role of indicating complex formation (i.e., shows a strong MEF response to the recognition event). Microwave irradiation rapidly accelerates PA deposition onto the surface ("rapid catch"), significantly speeding up the MEF-PA assay and resulting in a total assay run time of less than 40 min with an analytical sensitivity of less than 1 pg/ml PA.
    Analytical Biochemistry 03/2012; 425(1):54-61. · 2.31 Impact Factor
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    Anatoliy I. Dragan, Chris D. Geddes
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    ABSTRACT: Metal-enhanced fluorescence has attracted enormous research and commercial interest in recent years, due to the ability to significantly enhance fluorescence signatures in the near-field as well as protect fluorophores against photobleaching. In this article, we address one of the major unresolved questions, whether far-field fluorophore quantum yield, Q0, has a direct relationship to fluorescence enhancement factors in metal-enhanced fluorescence.
    Applied Physics Letters 03/2012; 100(9). · 3.52 Impact Factor
  • Yongxia Zhang, Buddha L Mali, Chris D Geddes
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    ABSTRACT: In this letter, we report the first observation of metal-enhanced exciplex fluorescence, observed from anthracene in the presence of diethylaniline. Anthracene in the presence of diethylaniline in close proximity to Silver Island Films (SIFs) shows enhanced monomer and exciplex emission as compared to a non-silvered control sample containing no silver nanoparticles. Our findings suggest two complementary methods for the enhancement: (i) surface plasmons can radiate coupled monomer and exciplex fluorescence efficiently, and (ii) enhanced absorption (enhanced electric near-field) further facilitates enhanced emission. Our exciplex studies help us to further understand the complex photophysics of the metal-enhanced fluorescence technology.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 01/2012; 85(1):134-8. · 2.13 Impact Factor

Publication Stats

5k Citations
750.35 Total Impact Points


  • 2010–2014
    • University of Maryland, Baltimore County
      • Department of Chemistry and Biochemistry
      Baltimore, Maryland, United States
    • GlaxoSmithKline plc.
      Londinium, England, United Kingdom
  • 2004–2010
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 2001–2010
    • University of Maryland, Baltimore
      • Department of Biochemistry and Molecular Biology
      Baltimore, MD, United States
  • 1998–2010
    • University of Strathclyde
      • Department of Physics
      Glasgow, Scotland, United Kingdom
  • 2009
    • University of California, Davis
      • Department of Physiology and Membrane Biology
      Davis, CA, United States
    • Umeå University
      • Department of Chemistry
      Umeå, Västerbotten, Sweden
    • University of Southern Denmark
      • Department of Biochemistry and Molecular Biology
      Copenhagen, Capital Region, Denmark
    • Georgia Institute of Technology
      Atlanta, Georgia, United States
    • Indian Association for the Cultivation of Science
      • Department of Physical Chemistry
      Calcutta, Bengal, India
    • University of Minho
      • Centro de Física
      Braga, Distrito de Braga, Portugal
    • VU University Amsterdam
      • Department of Analytical Chemistry and Spectroscopy
      Amsterdam, North Holland, Netherlands
    • Laval University
      • Département de Chimie
      Québec, Quebec, Canada
    • Saha Institute of Nuclear Physics
      Kolkata, Bengal, India
    • Montana State University
      • Department of Chemistry & Biochemistry
      Bozeman, MT, United States
  • 1998–2009
    • University of Wales
      • Department of Chemistry
      Cardiff, WLS, United Kingdom
  • 2003
    • University of Baltimore
      Baltimore, Maryland, United States
  • 1999–2001
    • Swansea University
      Swansea, Wales, United Kingdom